A. It is formed of two segments of a circle, and its centres have a radius
or line longer than the breadth of the arch, and may be described from an
acute-angled triangle. (fig. 5.).[TN-1]

Q. How is the equilateral arch formed and described?

A. From two segments of a circle; the centres of it have a radius or line
equal to the breadth of the arch, and it may be described from an
equilateral triangle. (fig. 6.)

[Illustration]

Q. How is the obtuse-angled arch formed and described?

A. Like the foregoing, it is formed from two segments of a circle, and the
centres of it have a radius shorter than the breadth of the arch; it is
described from an obtuse-angled triangle. (fig. 7.)

Q. During what period were these pointed arches in use?

A. They were all gradually introduced in the twelfth century, and
continued during the thirteenth century; after which the lancet arch
appears to have been generally discarded, though the other two prevailed
till a much later period.

Q. What are the different kinds of complex pointed arches?

A. Those commonly called the OGEE, or contrasted arch; and the TUDOR arch.

Q. How is the ogee, or contrasted arch, formed and described?

A. It is formed of four segments of a circle, and is described from four
centres, two placed within the arch on a level with the spring, and two
placed on the exterior of the arch, and level with the apex or point (fig.
8); each side is composed of a double curve, the lowermost convex and the
uppermost concave.

[Illustration]

Q. When was the ogee arch introduced, and how long did it prevail?

A. It was introduced early in the fourteenth century, and continued till
the close of the fifteenth century.

Q. How is the Tudor arch described?

A. From four centres; two on a level with the spring, and two at a
distance from it, and below. (fig. 9.)

Q. When was the Tudor arch introduced, and why is it so called?

A. It was introduced about the middle of the fifteenth century, or perhaps
earlier, but became most prevalent during the reigns of Henry the Seventh
and Henry the Eighth, under the Tudor dynasty, from which it derives its
name.

[Illustration]

Q. What other kinds of arches are there worthy of notice?

A. Those which are called foiled arches, as the round-headed trefoil (fig.
10), the pointed trefoil (fig. 11), and the square-headed trefoil (fig.
12). The first prevailed in the latter part of the twelfth and early part
of the thirt