timing belt pulleys: why is P.D. > O.D. ?

For timing belt pulleys, why is Pitch Diameter greater than Outside Diameter ? I would think it would be the opposite of that.
( I have been looking on the McMaster-Carr and Stock Drive Products sites. )
Thanks !
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The belt sticks out beyond the OD of the pulley.
Tim
--
Did I really still have that sig?

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wrote:

Outside
Products
Thanks! *Now* it makes sense! JCD
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