This article is from the Ballet and Modern Dance FAQ, by Tom Parsons firstname.lastname@example.org with numerous contributions by others.
Carlo Blasis (1797-1878) was the author of "The Theory and Practice
of the Art of Dancing" (1820) and "The Code of Terpsichore" (1823), an
expanded version of the earlier book. The material in these books is
virtually indistinguishable from ballet as it is taught to-day, and a
dancer of our own time could do worse than to read and follow his advice.
He requires a full 90-degree turnout, and his rules for placement are
essentially the same as ours: "Let your body be, in general, erect and
perpendicular on your legs.... Let your shoulders be low, your head high,
and your countenance animated and expressive." Dancers were now expected
to be able to extend the leg 90 degrees as a matter of course. Blasis's
description of pirouettes, which is too long to quote here, is as useful
to dancers to-day as it was in 1823.
He repeats Noverre's description of "arqu'e" and "jarret'e" and
enlarges on it; but he also describes the body types of the serious dancer
(what we would call the "danseur noble"), the "demi-caract`ere" dancer,
and the comic dancer. For the serious dancer he recommends particular
attention to the "adage" part of class; to him, "adage" is "the "ne plus
ultra" of our art" and "the touchstone of the dancer."