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+This file documents effors toward establishing a public API for iD, one that
+can support plugin development.
+
+## CSS selectors
+
+iD has a documented and stable set of classes that can be used to apply style or
+attach behavior to the visual representation of map data via CSS selectors.
+These classes relate to the vocabulary of the OSM data model, a related geometric
+vocabulary established by iD, and to the tags present on OSM entities.
+
+### OSM Data Model classes
+
+An SVG element on the map to which an iD.Entity has been bound as a datum shall
+have a class with that datum's type, i.e. either `.node` or `.way`. (If and when
+we add visual representations for relations, `.relation` may also be valid.)
+
+The visual representation of a single entity may be composed of several elements,
+e.g. ways are composed of casing and stroke. Such elements will have a distinct class
+identifying the particular aspect of representation, e.g. `.casing` and `.stroke`.
+
+The particular type of SVG element (`path`, `circle`, `image` etc.) that is used to
+implement that visual representation is explicitly NOT part of the public API. Avoid
+naming specific tags in CSS selectors; as iD evolves, we may need to change what SVG
+elements we use in order to implement a particular visual style.
+
+### Geometric classes
+
+In addition to the OSM element vocabulary of nodes, ways, and relations, iD has
+established a related geometric vocabulary consisting of points, lines, and areas.
+
+A **point** is a node that is either not a member of any way, or has specific tags
+that identify it as "interesting" in some way. For example, a node belonging to a
+way may also be considered a point if it has a `traffic_signal` tag. Elements
+representing points have a `.point` class. Since a point is always a node, they
+also have a `.node` class. Nodes that are not points can be selected using
+`.node:not(.point)` (TODO: come up with something better than that.)
+
+A **line** is a way that is not an area. Elements representing lines have a `.line`
+class. Since a line is also a way, they also have a `.way` class.
+
+An **area** is a way that is circular, has certain tags, or lacks certain other
+tags (see `iD.Way.isArea` for the exact definition). Elements representing areas
+have an `.area` class. Since an area is also a way, they also have a `.way` class.
+
+### Tag classes
+
+Elements also receive classes according to certain of the key-value tags that are
+assigned to them.
+
+TODO: elaborate.
+
+### Special classes
+
+A node that is a member of two or more ways shall have the `.shared` class. (TODO)
+
+Two or more nodes at identical coordinates shall each have an `.overlapped` class. (TODO)
+
+Elements comprising the entity currently under the cursor shall have the `.hover` class.
+(The `:hover` psuedo-class is insufficient when an entity's visual representation consists
+of several elements, only one of which can be `:hover`ed.)
+
+Elements that are currently active (being clicked or dragged) shall have the `.active`
+class.
+
+Elements that are currently selected shall have the `.selected` class.
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