(Lat. _complementum_, from _complere_, to fill up), that
which fills up or completes anything, e.g. the number of men necessary
to man a ship. In geometry, the complement of an angle is the difference
between the angle and a right angle; the complements of a parallelogram
are formed by drawing parallel to adjacent sides of a parallelogram two
lines intersecting on a diagonal; four parallelograms are thus formed,
and the two not about the diagonal of the original parallelogram are the
complements of the parallelogram. In analysis, a complementary function
is a partial solution to a differential equation (q.v.); complementary
operators are reciprocal or inverse operators, i.e. two operations A and
B are complementary when both operating on the same figure or function
leave it unchanged. A "complementary colour" is one which produces white
when mixed with another (see COLOUR). In Spanish the word _cumplimento_
was used in a particular sense of the fulfilment of the duties of polite
behaviour and courtesy, and it came through the French and Italian forms
into use in English, with a change in spelling to "compliment," with the
sense of an act of politeness, especially of a polite expression of
praise, or of social regard and greetings. The word "comply," meaning
to act in accordance with wishes, orders or conditions, is also derived
from the same origin, but in sense is connected with "ply" or "pliant,"
from Lat. _plicare_, to bend, with the idea of subserviently yielding to
the wishes of another.

COMPLUVIUM (from Lat. _compluere_, to flow together, i.e. in reference
to the rain being collected and falling through), in architecture, the
Latin term for the open space left in the roof of the atrium of a Roman
house for lighting it and the rooms round (see CAVAEDIUM).

COMPOSITAE, the name given to the largest natural order of flowering
plants, containing about one-tenth of the whole number and characterized
by the crowding of the flowers into heads. The order is cosmopolitan,
and the plants show considerable variety in habit. The great majority,
including most British representatives, are herbaceous, but in the
warmer parts of the world shrubs and arborescent forms also occur; the
latter are characteristic of the flora of oceanic islands. In herbaceous
plants the leaves are often arranged in a rosette on a much shortened
stem, as in dandelion, daisy and others; when the stem is elongated the
leaves are generally