The Complementary angles reference article from the English Wikipedia on 24-Apr-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Complementary angles

A pair of angles are complementary if their respective measures sum to 90 degrees.

If the two complementary angles are adjacent (i.e., have a common vertex and share a side, but do not have any interior points in common) their non-shared sides form a right angle.

In Euclidean geometry, the two acute angles in a right triangle are complementary, because there are 180° in a triangle and 90° have been accounted for by the right angle.

The adjective "complementary" comes from the Latin complementum, which derives from the fourth form of the Latin verb (compleo:I fill up, complere:to fill up, complevi:I filled up, completum: having been filled up). An acute angle is filled up by its complement to form a right angle, which is the most basic kind of angle, because it can be constructed by bisecting a line. A right angle is a complete (full) angle.

See also: supplementary angles.

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