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.
Roman arch (True Arch) - An arch made of voissoirs and a keystone
(p 22 see keystone).
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.
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