# Coordinates, Points and Rectangles

The coordinate system of a Window or View has (0, 0) at the top left corner,
with *x* coordinates increasing to the right and *y* coordinates
increasing downward. The coordinates label infinitely thin horizontal and
vertical lines, and pixels exist between these lines, as illustrated in the
figure below.

A **point** is represented by a 2-tuple (*x, y*). A 2-tuple can also
be used to represent as size (*width, height*) or a relative displacement
(*dx, dy*).

A **rectangle** is represented by a 4-tuple (*left, top, right, bottom*).
If *right* <= *left* or *bottom* <= *top*, the
rectangle is an *empty rectangle* and is considered to enclose no pixels.

**Angles** are measured in degrees *anticlockwise* from the *x*-axis.

The Geometry submodule provides a collection
of functions for operating on points and rectangles.

Relationship between coordinates,
pixels, points and rectangles