The Imperial Capital (Capitalis Minumentalis) was the letter used on the monuments of Ancient Rome to proclaim the might of the Roman Empire, and is indisputably the most stately of all scripts. The earliest examples of a mature Imperial letter date from the first century B.C., and some of the finest models are inscribed on the base of Trajan Column in Rome (opposite). These stone-cut letters were carved directly on top of brush-draws forms, their proportions dictated by the natural movement of the hand.
Capital Letters with serifs had been written by the Greeks from the fourth century B.C., However, it was only when the Romans developed a springy, broad-edge brush from the hairs of the red sable that it became technically possible to draw serifs and other letter parts quickly and with precision. When used within the natural compass of the hand, this tool proved crucial in determining the shape of the Imperial Capital itself.
(David Harris in The art of calligraphy. A practical guide to skill and techniques p.108, Dorling Kindersley, New York.)