both groin and quoin are wedges...quoins are corner
blocks-- wedges of different sort

(also "coign")

Additional meaning: in the Good Old Days, when type was set by hand out of the California Job Case, the block of carefully-ordered type was placed into a form (the "chase"), the rest of the space filled out with blocks, and then the whole page was firmly fixed in place by tightening wedge-shaped pieces against each other, one set horizontal and another vertical, to use up any remaining space. The wedges were called - you guessed it - quoins, and the gadget used to do the final tightening was a "quoin key."

(And if you were so unfortunate as to drop the form before it was rigid, everything fell apart - the type was now "pied" - and you had to start all over again, and you were QUITE unhappy about all that wasted effort.)

If I can find a URL for the quoin, I'll add it on.

edit: Durn. Lots of places with words to describe the process, but I couldn't find one with a picture.