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+Apertures in Gerbonara
+======================
+
+Gerbonara maps all standard Gerber apertures to subclasses of the Aperture_ class. These subclasses: CircleAperture_,
+RectangleAperture_, ObroundAperture_ and PolygonAperture_. Aperture macro instantiations get mapped to
+ApertureMacroInstance_ (also an Aperture_ subclass).
+
+All Aperture_ subclasses have these common attributes:
+
+
+`hole_dia`
+    float with diameter of hole. 0 for no hole.
+
+`hole_rect_h`
+    float or None. If not None, specifies a rectangular hole of size `hole_dia * hole_rect_h` instead of a round hole.
+
+`unit`
+    LengthUnit_ for all of this aperture's fields
+
+`attrs`
+    GerberX2 attributes of this aperture. Note that this will only contain aperture attributes, not file attributes.
+    File attributes are stored in the `attrs` of GerberFile_.
+
+`original_number`
+    int of aperture index this aperture had when it was read from the Gerber file. This field is purely informational
+    since apertures are de-duplicated and re-numbered when writing a Gerber file. For `D10`, this field would be `10`.
+    If you programmatically create a new aperture, you do not have to set this.
+
+`rotation`
+    Aperture rotation in radians counter-clockwise. This field is not part of the Gerber standard. Standard rectangle
+    and obround apertures do not support rotation. Gerbonara converts rotated apertures into aperture macros during
+    Gerber export as necessary.
+
+CircleAperture
+--------------
+
+This is the only one valid for use in Line_ or Arc_.
+
+Attributes:
+
+Common attributes:
+    `hole_dia`, `hole_rect_h`, `unit`, `attrs`, and `original_number`. `rotation` is present but has no effect in
+    CircleAperture_.
+
+`diameter`
+    float with diameter of aperture in the unit from the aperture's `unit` field. 
+    
+RectangleAperture
+-----------------
+
+Common attributes:
+    `hole_dia`, `hole_rect_h`, `unit`, `attrs`, `original_number`, and `rotation`
+
+`w`, `h`
+    floats with width or height of rectangle in units from the aperture's `unit` field. 
+
+ObroundAperture
+---------------
+
+Aperture whose shape is the convex hull of two circles of equal radii.
+
+Common attributes:
+    `hole_dia`, `hole_rect_h`, `unit`, `attrs`, `original_number`, and `rotation`
+
+`w`, `h`
+    floats with width and height of bounding box of obround. The smaller one of these will be the diameter of the
+    obround's ends. If `w` is larger, the result will be a landscape obround. If `h` is larger, it will be a portrait
+    obround.
+
+PolygonAperture
+---------------
+
+Aperture whose shape is a regular n-sided polygon (e.g. pentagon, hexagon etc.).
+
+
+Common attributes:
+    `hole_dia`, `unit`, `attrs`, `original_number`, and `rotation`. `hole_rect_h` is not supported in PolygonAperture_
+    since the Gerber spec does not list it.
+
+`diameter`
+    float with diameter of circumscribing circle, i.e. the circle that all the polygon's corners lie on.
+
+`n_vertices`
+    int with number of corners of this polygon. Three for a triangle, four for a square, five for a pentagon etc.
+
+ApertureMacroInstance
+---------------------
+
+One instance of an aperture macro. An aperture macro defined with an `AM` statement can be instantiated by multiple `AD`
+aperture definition statements using different parameters. An ApertureMacroInstance_ is one such binding of a macro to a
+particular set of parameters. Note that you still need an ApertureMacroInstance_ even if your ApertureMacro_ has no
+parameters since an ApertureMacro_ is not an Aperture_ by itself.
+
+Attributes:
+
+Common attributes:
+    `unit`, `attrs`, `original_number`, and `rotation`. ApertureMacroInstance_ does not support `hole_dia` or
+    `hole_rect_h`. `rotation` is handled by re-writing the ApertureMacro_ during export.
+
+`macro`
+    The ApertureMacro_ that is bound here
+
+`parameters`
+    list of ints or floats with the parameters for this macro. The first element is `$1`, the second is `$2` etc.
+
+ExcellonTool
+------------
+
+Special Aperture_ subclass for use in ExcellonFile_. Similar to CircleAperture_, but does not have `hole_dia` or
+`hole_rect_h`, and has additional `plated` and `depth_offset` attributes.
+
+
+Common attributes:
+    `unit`, `original_number`
+
+`plated`
+    bool or None. True if this hole/slot is copper-plated, False if not, and None if it is undefined or unknown.
+
+`depth_offset`
+    float with Excellon depth offset for this hole or slot. If the fab supports this, this can be used to create
+    features that do not go all the way through the board.
+
+Aperture generalization
+-----------------------
+
+Gerbonara supports rotating both individual graphic objects and whole files. Alas, this was not a use case that was
+intended when the Gerber format was developed. We can rotate lines, arcs, and regions alright by simply rotatint all of
+their points. Flashes are where things get tricky: Individual flashes cannot be rotated at all in any widely supported
+way. There are some newer additions to the standard, but I would be surprised if any of the cheap board houses
+understand those. The only way to rotate a flash is to rotate the aperture, not the flash. For cirlces, this is a no-op.
+For polygons, we simply change the angle parameter. However, for rectangles and obrounds this gets tricky: Neither one
+supports a rotation parameter. The only way to rotate these is to convert them to an aperture macro, then rotate that.
+
+This behavior of using aperture macros for general rotated rectangles is common behavior among CAD tools. Gerbonara adds
+a non-standard `rotation` attribute to all apertures except CircleAperture_ and transparently converts rotated instances
+to the appropriate ApertureMacroInstance_ objects while it writes out the file. Be aware that this may mean that an
+object that in memory has a RectangleAperture_ might end up with an aperture macro instance in the output Gerber file.
+