diff --git a/masque/__init__.py b/masque/__init__.py
index a2d138f..c22c18c 100644
--- a/masque/__init__.py
+++ b/masque/__init__.py
@@ -37,7 +37,7 @@ from .pattern import Pattern
 from .library import Library, MutableLibrary, WrapROLibrary, WrapLibrary, LazyLibrary, AbstractView
 from .ports import Port, PortList
 from .abstract import Abstract
-from .builder import Builder, Tool
+from .builder import Builder, Tool, FlatBuilder
 from .utils import ports2data
 
 
diff --git a/masque/builder/__init__.py b/masque/builder/__init__.py
index 27cc0f9..c5a6103 100644
--- a/masque/builder/__init__.py
+++ b/masque/builder/__init__.py
@@ -1,3 +1,4 @@
 from .builder import Builder
+from .flatbuilder import FlatBuilder
 from .utils import ell
 from .tools import Tool
diff --git a/masque/builder/flatbuilder.py b/masque/builder/flatbuilder.py
new file mode 100644
index 0000000..8088a4a
--- /dev/null
+++ b/masque/builder/flatbuilder.py
@@ -0,0 +1,529 @@
+from typing import Dict, Tuple, Union, TypeVar, Optional, Sequence
+from typing import MutableMapping, Mapping
+import copy
+import logging
+
+import numpy
+from numpy import pi
+from numpy.typing import ArrayLike
+
+from ..pattern import Pattern
+from ..error import PortError, BuildError
+from ..ports import PortList, Port
+from ..utils import SupportsBool
+from .tools import Tool
+from .utils import ell
+
+
+logger = logging.getLogger(__name__)
+
+
+BB  = TypeVar('BB',  bound='FlatBuilder')
+
+
+class FlatBuilder(PortList):
+    """
+    TODO DOCUMENT FlatBuilder
+    """
+    __slots__ = ('pattern', 'tools', '_dead')
+
+    pattern: Pattern
+    """ Layout of this device """
+
+    tools: Dict[Optional[str], Tool]
+    """
+    Tool objects are used to dynamically generate new single-use Devices
+    (e.g wires or waveguides) to be plugged into this device.
+    """
+
+    _dead: bool
+    """ If True, plug()/place() are skipped (for debugging)"""
+
+    @property
+    def ports(self) -> Dict[str, Port]:
+        return self.pattern.ports
+
+    @ports.setter
+    def ports(self, value: Dict[str, Port]) -> None:
+        self.pattern.ports = value
+
+    def __init__(
+            self,
+            *,
+            pattern: Optional[Pattern] = None,
+            ports: Union[None, Mapping[str, Port]] = None,
+            tools: Union[None, Tool, MutableMapping[Optional[str], Tool]] = None,
+            ) -> None:
+        """
+        # TODO documentation for FlatBuilder() constructor
+        """
+        if pattern is not None:
+            self.pattern = pattern
+        else:
+            self.pattern = Pattern()
+
+        if ports is not None:
+            if self.pattern.ports:
+                raise BuildError('Ports supplied for pattern with pre-existing ports!')
+            self.pattern.ports.update(copy.deepcopy(dict(ports)))
+
+        if tools is None:
+            self.tools = {}
+        elif isinstance(tools, Tool):
+            self.tools = {None: tools}
+        else:
+            self.tools = dict(tools)
+
+        self._dead = False
+
+    @classmethod
+    def interface(
+            cls,
+            source: Union[PortList, Mapping[str, Port]],
+            *,
+            tools: Union[None, Tool, MutableMapping[Optional[str], Tool]] = None,
+            in_prefix: str = 'in_',
+            out_prefix: str = '',
+            port_map: Optional[Union[Dict[str, str], Sequence[str]]] = None,
+            ) -> 'FlatBuilder':
+        """
+        Begin building a new device based on all or some of the ports in the
+          source device. Do not include the source device; instead use it
+          to define ports (the "interface") for the new device.
+
+        The ports specified by `port_map` (default: all ports) are copied to
+          new device, and additional (input) ports are created facing in the
+          opposite directions. The specified `in_prefix` and `out_prefix` are
+          prepended to the port names to differentiate them.
+
+        By default, the flipped ports are given an 'in_' prefix and unflipped
+          ports keep their original names, enabling intuitive construction of
+          a device that will "plug into" the current device; the 'in_*' ports
+          are used for plugging the devices together while the original port
+          names are used for building the new device.
+
+        Another use-case could be to build the new device using the 'in_'
+          ports, creating a new device which could be used in place of the
+          current device.
+
+        Args:
+            source: A collection of ports (e.g. Pattern, Builder, or dict)
+                from which to create the interface.
+            library: Used for buildin functions; if not passed and the source
+            library: Library from which existing patterns should be referenced,
+                and to which new ones should be added. If not provided,
+                the source's library will be used (if available).
+            tools: Tool objects are used to dynamically generate new single-use
+                patterns (e.g wires or waveguides) while building. If not provided,
+                the source's tools will be reused (if available).
+            in_prefix: Prepended to port names for newly-created ports with
+                reversed directions compared to the current device.
+            out_prefix: Prepended to port names for ports which are directly
+                copied from the current device.
+            port_map: Specification for ports to copy into the new device:
+                - If `None`, all ports are copied.
+                - If a sequence, only the listed ports are copied
+                - If a mapping, the listed ports (keys) are copied and
+                    renamed (to the values).
+
+        Returns:
+            The new builder, with an empty pattern and 2x as many ports as
+              listed in port_map.
+
+        Raises:
+            `PortError` if `port_map` contains port names not present in the
+                current device.
+            `PortError` if applying the prefixes results in duplicate port
+                names.
+        """
+
+        if tools is None and hasattr(source, 'tools') and isinstance(source.tools, dict):
+            tools = source.tools
+
+        if isinstance(source, PortList):
+            orig_ports = source.ports
+        elif isinstance(source, dict):
+            orig_ports = source
+        else:
+            raise BuildError(f'Unable to get ports from {type(source)}: {source}')
+
+        if port_map:
+            if isinstance(port_map, dict):
+                missing_inkeys = set(port_map.keys()) - set(orig_ports.keys())
+                mapped_ports = {port_map[k]: v for k, v in orig_ports.items() if k in port_map}
+            else:
+                port_set = set(port_map)
+                missing_inkeys = port_set - set(orig_ports.keys())
+                mapped_ports = {k: v for k, v in orig_ports.items() if k in port_set}
+
+            if missing_inkeys:
+                raise PortError(f'`port_map` keys not present in source: {missing_inkeys}')
+        else:
+            mapped_ports = orig_ports
+
+        ports_in = {f'{in_prefix}{name}': port.deepcopy().rotate(pi)
+                    for name, port in mapped_ports.items()}
+        ports_out = {f'{out_prefix}{name}': port.deepcopy()
+                     for name, port in mapped_ports.items()}
+
+        duplicates = set(ports_out.keys()) & set(ports_in.keys())
+        if duplicates:
+            raise PortError(f'Duplicate keys after prefixing, try a different prefix: {duplicates}')
+
+        new = FlatBuilder(ports={**ports_in, **ports_out}, tools=tools)
+        return new
+
+    def plug(
+            self: BB,
+            other: Pattern,
+            map_in: Dict[str, str],
+            map_out: Optional[Dict[str, Optional[str]]] = None,
+            *,
+            mirrored: Tuple[bool, bool] = (False, False),
+            inherit_name: bool = True,
+            set_rotation: Optional[bool] = None,
+            ) -> BB:
+        """
+        Instantiate another pattern into the current device, connecting
+          the ports specified by `map_in` and renaming the unconnected
+          ports specified by `map_out`.
+
+        Examples:
+        =========
+        - `my_device.plug(lib, 'subdevice', {'A': 'C', 'B': 'B'}, map_out={'D': 'myport'})`
+            instantiates `lib['subdevice']` into `my_device`, plugging ports 'A' and 'B'
+            of `my_device` into ports 'C' and 'B' of `subdevice`. The connected ports
+            are removed and any unconnected ports from `subdevice` are added to
+            `my_device`. Port 'D' of `subdevice` (unconnected) is renamed to 'myport'.
+
+        - `my_device.plug(lib, 'wire', {'myport': 'A'})` places port 'A' of `lib['wire']`
+            at 'myport' of `my_device`.
+            If `'wire'` has only two ports (e.g. 'A' and 'B'), no `map_out` argument is
+            provided, and the `inherit_name` argument is not explicitly set to `False`,
+            the unconnected port of `wire` is automatically renamed to 'myport'. This
+            allows easy extension of existing ports without changing their names or
+            having to provide `map_out` each time `plug` is called.
+
+        Args:
+            other: An `Abstract` describing the device to be instatiated.
+            map_in: Dict of `{'self_port': 'other_port'}` mappings, specifying
+                port connections between the two devices.
+            map_out: Dict of `{'old_name': 'new_name'}` mappings, specifying
+                new names for ports in `other`.
+            mirrored: Enables mirroring `other` across the x or y axes prior
+                to connecting any ports.
+            inherit_name: If `True`, and `map_in` specifies only a single port,
+                and `map_out` is `None`, and `other` has only two ports total,
+                then automatically renames the output port of `other` to the
+                name of the port from `self` that appears in `map_in`. This
+                makes it easy to extend a device with simple 2-port devices
+                (e.g. wires) without providing `map_out` each time `plug` is
+                called. See "Examples" above for more info. Default `True`.
+            set_rotation: If the necessary rotation cannot be determined from
+                the ports being connected (i.e. all pairs have at least one
+                port with `rotation=None`), `set_rotation` must be provided
+                to indicate how much `other` should be rotated. Otherwise,
+                `set_rotation` must remain `None`.
+
+        Returns:
+            self
+
+        Raises:
+            `PortError` if any ports specified in `map_in` or `map_out` do not
+                exist in `self.ports` or `other_names`.
+            `PortError` if there are any duplicate names after `map_in` and `map_out`
+                are applied.
+            `PortError` if the specified port mapping is not achieveable (the ports
+                do not line up)
+        """
+        if self._dead:
+            logger.error('Skipping plug() since device is dead')
+            return self
+
+        # If asked to inherit a name, check that all conditions are met
+        if (inherit_name
+                and not map_out
+                and len(map_in) == 1
+                and len(other.ports) == 2):
+            out_port_name = next(iter(set(other.ports.keys()) - set(map_in.values())))
+            map_out = {out_port_name: next(iter(map_in.keys()))}
+
+        if map_out is None:
+            map_out = {}
+        map_out = copy.deepcopy(map_out)
+
+        self.check_ports(other.ports.keys(), map_in, map_out)
+        translation, rotation, pivot = self.find_transform(
+            other,
+            map_in,
+            mirrored=mirrored,
+            set_rotation=set_rotation,
+            )
+
+        # get rid of plugged ports
+        for ki, vi in map_in.items():
+            del self.ports[ki]
+            map_out[vi] = None
+
+        self.place(other, offset=translation, rotation=rotation, pivot=pivot,
+                   mirrored=mirrored, port_map=map_out, skip_port_check=True)
+        return self
+
+    def place(
+            self: BB,
+            other: Pattern,
+            *,
+            offset: ArrayLike = (0, 0),
+            rotation: float = 0,
+            pivot: ArrayLike = (0, 0),
+            mirrored: Tuple[bool, bool] = (False, False),
+            port_map: Optional[Dict[str, Optional[str]]] = None,
+            skip_port_check: bool = False,
+            ) -> BB:
+        """
+        Instantiate the device `other` into the current device, adding its
+          ports to those of the current device (but not connecting any ports).
+
+        Mirroring is applied before rotation; translation (`offset`) is applied last.
+
+        Examples:
+        =========
+        - `my_device.place(pad, offset=(10, 10), rotation=pi / 2, port_map={'A': 'gnd'})`
+            instantiates `pad` at the specified (x, y) offset and with the specified
+            rotation, adding its ports to those of `my_device`. Port 'A' of `pad` is
+            renamed to 'gnd' so that further routing can use this signal or net name
+            rather than the port name on the original `pad` device.
+
+        Args:
+            other: An `Abstract` describing the device to be instatiated.
+            offset: Offset at which to place the instance. Default (0, 0).
+            rotation: Rotation applied to the instance before placement. Default 0.
+            pivot: Rotation is applied around this pivot point (default (0, 0)).
+                Rotation is applied prior to translation (`offset`).
+            mirrored: Whether theinstance should be mirrored across the x and y axes.
+                Mirroring is applied before translation and rotation.
+            port_map: Dict of `{'old_name': 'new_name'}` mappings, specifying
+                new names for ports in the instantiated device. New names can be
+                `None`, which will delete those ports.
+            skip_port_check: Can be used to skip the internal call to `check_ports`,
+                in case it has already been performed elsewhere.
+
+        Returns:
+            self
+
+        Raises:
+            `PortError` if any ports specified in `map_in` or `map_out` do not
+                exist in `self.ports` or `other.ports`.
+            `PortError` if there are any duplicate names after `map_in` and `map_out`
+                are applied.
+        """
+        if self._dead:
+            logger.error('Skipping place() since device is dead')
+            return self
+
+        if port_map is None:
+            port_map = {}
+
+        if not skip_port_check:
+            self.check_ports(other.ports.keys(), map_in=None, map_out=port_map)
+
+        ports = {}
+        for name, port in other.ports.items():
+            new_name = port_map.get(name, name)
+            if new_name is None:
+                continue
+            ports[new_name] = port
+
+        for name, port in ports.items():
+            p = port.deepcopy()
+            p.mirror2d(mirrored)
+            p.rotate_around(pivot, rotation)
+            p.translate(offset)
+            self.ports[name] = p
+
+        other_copy = other.deepcopy()
+        other_copy.mirror2d(mirrored)
+        other_copy.rotate_around(pivot, rotation)
+        other_copy.translate_elements(offset)
+        self.pattern.append(other_copy)
+        return self
+
+    def translate(self: BB, offset: ArrayLike) -> BB:
+        """
+        Translate the pattern and all ports.
+
+        Args:
+            offset: (x, y) distance to translate by
+
+        Returns:
+            self
+        """
+        self.pattern.translate_elements(offset)
+        for port in self.ports.values():
+            port.translate(offset)
+        return self
+
+    def rotate_around(self: BB, pivot: ArrayLike, angle: float) -> BB:
+        """
+        Rotate the pattern and all ports.
+
+        Args:
+            angle: angle (radians, counterclockwise) to rotate by
+            pivot: location to rotate around
+
+        Returns:
+            self
+        """
+        self.pattern.rotate_around(pivot, angle)
+        for port in self.ports.values():
+            port.rotate_around(pivot, angle)
+        return self
+
+    def mirror(self: BB, axis: int) -> BB:
+        """
+        Mirror the pattern and all ports across the specified axis.
+
+        Args:
+            axis: Axis to mirror across (x=0, y=1)
+
+        Returns:
+            self
+        """
+        self.pattern.mirror(axis)
+        for p in self.ports.values():
+            p.mirror(axis)
+        return self
+
+    def set_dead(self: BB) -> BB:
+        """
+        Disallows further changes through `plug()` or `place()`.
+        This is meant for debugging:
+        ```
+            dev.plug(a, ...)
+            dev.set_dead()      # added for debug purposes
+            dev.plug(b, ...)    # usually raises an error, but now skipped
+            dev.plug(c, ...)    # also skipped
+            dev.pattern.visualize()     # shows the device as of the set_dead() call
+        ```
+
+        Returns:
+            self
+        """
+        self._dead = True
+        return self
+
+    def __repr__(self) -> str:
+        s = f'<FlatBuilder {self.pattern} >'
+        return s
+
+    def retool(
+            self: BB,
+            tool: Tool,
+            keys: Union[Optional[str], Sequence[Optional[str]]] = None,
+            ) -> BB:
+        if keys is None or isinstance(keys, str):
+            self.tools[keys] = tool
+        else:
+            for key in keys:
+                self.tools[key] = tool
+        return self
+
+    def path(
+            self: BB,
+            portspec: str,
+            ccw: Optional[SupportsBool],
+            length: float,
+            *,
+            tool_port_names: Sequence[str] = ('A', 'B'),
+            base_name: str = '_path_',
+            **kwargs,
+            ) -> BB:
+        if self._dead:
+            logger.error('Skipping path() since device is dead')
+            return self
+
+        tool = self.tools.get(portspec, self.tools[None])
+        in_ptype = self.pattern[portspec].ptype
+        pat = tool.path(ccw, length, in_ptype=in_ptype, port_names=tool_port_names, **kwargs)
+        return self.plug(pat, {portspec: tool_port_names[0]})
+
+    def path_to(
+            self: BB,
+            portspec: str,
+            ccw: Optional[SupportsBool],
+            position: float,
+            *,
+            tool_port_names: Sequence[str] = ('A', 'B'),
+            base_name: str = '_pathto_',
+            **kwargs,
+            ) -> BB:
+        if self._dead:
+            logger.error('Skipping path_to() since device is dead')
+            return self
+
+        port = self.pattern[portspec]
+        x, y = port.offset
+        if port.rotation is None:
+            raise PortError(f'Port {portspec} has no rotation and cannot be used for path_to()')
+
+        if not numpy.isclose(port.rotation % (pi / 2), 0):
+            raise BuildError('path_to was asked to route from non-manhattan port')
+
+        is_horizontal = numpy.isclose(port.rotation % pi, 0)
+        if is_horizontal:
+            if numpy.sign(numpy.cos(port.rotation)) == numpy.sign(position - x):
+                raise BuildError(f'path_to routing to behind source port: x={x:g} to {position:g}')
+            length = numpy.abs(position - x)
+        else:
+            if numpy.sign(numpy.sin(port.rotation)) == numpy.sign(position - y):
+                raise BuildError(f'path_to routing to behind source port: y={y:g} to {position:g}')
+            length = numpy.abs(position - y)
+
+        return self.path(portspec, ccw, length, tool_port_names=tool_port_names, base_name=base_name, **kwargs)
+
+    def mpath(
+            self: BB,
+            portspec: Union[str, Sequence[str]],
+            ccw: Optional[SupportsBool],
+            *,
+            spacing: Optional[Union[float, ArrayLike]] = None,
+            set_rotation: Optional[float] = None,
+            tool_port_names: Sequence[str] = ('A', 'B'),
+            force_container: bool = False,
+            base_name: str = '_mpath_',
+            **kwargs,
+            ) -> BB:
+        if self._dead:
+            logger.error('Skipping mpath() since device is dead')
+            return self
+
+        bound_types = set()
+        if 'bound_type' in kwargs:
+            bound_types.add(kwargs['bound_type'])
+            bound = kwargs['bound']
+        for bt in ('emin', 'emax', 'pmin', 'pmax', 'min_past_furthest'):
+            if bt in kwargs:
+                bound_types.add(bt)
+                bound = kwargs[bt]
+
+        if not bound_types:
+            raise BuildError('No bound type specified for mpath')
+        elif len(bound_types) > 1:
+            raise BuildError(f'Too many bound types specified for mpath: {bound_types}')
+        bound_type = tuple(bound_types)[0]
+
+        if isinstance(portspec, str):
+            portspec = [portspec]
+        ports = self.pattern[tuple(portspec)]
+
+        extensions = ell(ports, ccw, spacing=spacing, bound=bound, bound_type=bound_type, set_rotation=set_rotation)
+
+        if len(ports) == 1 and not force_container:
+            # Not a bus, so having a container just adds noise to the layout
+            port_name = tuple(portspec)[0]
+            return self.path(port_name, ccw, extensions[port_name], tool_port_names=tool_port_names)
+        else:
+            bld = FlatBuilder.interface(source=ports, tools=self.tools)
+            for port_name, length in extensions.items():
+                bld.path(port_name, ccw, length, tool_port_names=tool_port_names)
+            return self.plug(bld.pattern, {sp: 'in_' + sp for sp in ports.keys()})       # TODO safe to use 'in_'?