Where is the hash key on my MacBook Pro?

After looking to post under a certain hash-tag on Twitter the other day, it occurred to me that my MacBook Pro had no hash key.  Strange, so I checked and double checked and there was definitely no hash key.  So how do you enter a hash symbol on a MacBook Pro?  It has to be done by holding the alt key + 3  (alt+3) then there you have it a hash symbol # hooray!