This article is from the Italy FAQ, by Gianluigi Sartori firstname.lastname@example.org, Paolo Fiorini email@example.com with numerous contributions by others.
Paolo Bellutta <bellutta@ohsu.EDU> writes:
The only difference I found was the optical axis of a cylindrical
lens (for astigmatism) have different reference and direction.
In the US. 0 deg. is like in trigonometry while in Italy the
right eye has the 0 deg. on the horizontal axis, but on the
*left*, and positive clockwise.