A "true arch" uses a keystone, which is tapered from top to bottom. That allows a smooth, round curve. The corbel arch was a predecessor to that.
The corbel was adapted for use in interiors and on furniture, and was popular in Renaissance designs.
Roman arch (True Arch) - An arch made of voissoirs and a keystone
(p 22 see keystone).
segmental arch - a section of arch that equals the arc of a circle less than a half circle (p.22-all window arches, p.24-all upper floor window arches, p.28-first floor porch arches).
It is not unusual to find a keystone incorporated, on the model of a true arch; but as the forces in a lintel (compression at the top and tension at the bottom) bear no resemblance the forces in an arch proper (all members in compression) the result is structurally impure.
Pugin and his followers were convinced that the only true architecture for northern climates was medieval architecture.
McMansions have no true architectural style, but borrow motifs and elements from various and often incompatible architectural styles, using tract-home construction to mimic traditionally-built homes. Large homes crowded onto small lots also block sunlight, putting nearby residences into shadow.
Masonry constructed over a wall opening by a series of courses projecting from each side and stepped progressively further forward until they meet at midpoint; not a true arch.
See also: House, Architecture, Ornament, Gothic, Floor