dado: Lower part of an interior wall, often decorated with arcading.

depressed arch: A flattened arch, slightly pointed on top. It appears in Late Gothic of the fifteenth and sixteenth centuries.

See also arch.
Compare with other types of arches.

diagonal ribs: The moldings which mark the diagonals in a rib vault.

See also rib vault
Other types of ribs: lierne, ridge, tierceron, transverse

diaper: A pattern formed by small, repeated geometrical motifs set adjacent to one another, used to decorate stone surfaces in architecture and as a background to illuminations in manuscripts, wall painting or panel painting.

donjon or keep : A freestanding defense tower in a castle complex.
Compare with motte-and-bailey

dog tooth: An ornamental motif consisting of a square, four-leafed figure, the center of which projects in a point. It was a very populare in Early English (Early Gothic) architecture.

Compare with other repetative decorative motifs.

dome: A hemispherical vault.

See also semi-dome, squinch, pendentive

drum: A cylindrical wall which supports a dome.

drum pier: Massive circular support.

See also: pier, alternation of support
Other types of piers: composite