This article is from the Ballet and Modern Dance FAQ, by Tom Parsons email@example.com with numerous contributions by others.
For greater freedom of movement. Many of the steps in ballet are
done with the leg extended; the kicks we associate with a chorus line are
like this. For various reasons having to do with the structure of the hip
joint, a dancer can obtain the greatest extension if the leg is rotated
outward, away from its usual position. This rotation means that you can
move to the side as readily as to the front or back.
You also frequently need to change the position of the feet, from
right foot in front to left foot in front or vice versa. One of the most
elementary jumps, called a "changement de pieds" ("change of the feet";
"changement" for short), consists of nothing else: jump up and land with
the other foot in front. These changes must be made very quickly, and
again they can be done most easily if the feet are pointed in opposite
This position of the legs is known as turnout, and it is probably
the most conspicuous aspect of balletic posture. As this description
implies, it is mostly a practical measure, although it may be done for
appearance as well. In the first ballets, the dancers performed in the
middle of the hall, surrounded on all sides by the audience. When ballet
moved to the proscenium stage, in the middle of the seventeenth century,
men began to dance turned out. This has led historians to suggest that
turnout originated because it looked better on stage. But it may have been
because extension showed to better advantage on the stage and that dancers
turned out for the sake of greater extension.
Turnout does not begin from the ankles. You do not force your feet
into that position and let everything from there on up follow. Turnout
begins at the hip joint, and it is better to be turned out imperfectly from
the hip than to strain the joints at the ankles and knees. Indeed, few
people can turn out perfectly, with the feet pointing in exactly opposite
directions, unless they have started as children (and sometimes not even
then), and boys are not expected to be as turned out as girls are.
If you were going to select one thing that sets ballet off from
every other kind of dancing (not a good idea, but suppose you had to) it
would probably be turnout. Dancers sometimes say that you turn out your
entire body. Physically, this is impossible--the ribs are firmly attached
to the breastbone, after all--but that describes the way it *feels*. It is
most visible in the feet, but it originates from the hips, and sometimes
seems to originate from even higher than that. There's an openness to the
dancer's whole body in ballet.
For additional information, see the file, why-turnout-in-ballet.txt
in the Dancers' Archive.