This article is from the Ergonomics and Typing Injury FAQ, by Dan Wallach email@example.com with numerous contributions by others.
A. The elbows should form a 90 degree angle while *hanging* at your sides
from the shoulders. Rarely do chairs with armrests allow this position. It
is *very* important that the shoulders remain relaxed in a lowered
position during keyboarding (see #6).