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If you google NBA basketball rules book you can get a .pdf of them (at least a relativley recent one).
Note that it's only illegal if the offensive player causes undue contact. The rule seems to be roughly similar to NFHS and NCAA rules.
Here's from 2013-14 (sorry about the poor cut-and-paste -- whatever pdf reader I'm working with here isn't very good):
Section III—By Screening
A player who sets a screen shall not (1) assume a p
osition nearer than a normal step
from an opponent, if that opponent is stationary an
d unaware of the screener’s position, or
(2) make illegal contact with an opponent when he a
opportunity to stop and/or change direction before
making illegal contact, or (4) move later-
ally or toward an opponent being screened, after ha
ving assumed a legal position. The
screener may move in the same direction and path of
the opponent being screened.
In (3) above, the speed of the opponent being scree
ned will determine what the
screener’s stationary position may be. This positio
n will vary and may be one to two normal
steps or strides from his opponen
ssumes a position at the side or front of
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