Help me decipher the utility of a "CATS" bond.

My grandmother recently passed away and in her possession we discovered a matured "CATS" (Certificate of Accrual on Treasury Securities) "Series P" bond to be paid to my name. It's nothing huge, a few thousand dollars, but it matured in 2002. Though I'm fairly sure I've figured out how to redeem it, I'm having absolutely no success in understanding exactly what a "CATS" bond is. Plenty of Google results but it's all Greek to me, quite frankly. [more inside]
posted by incomple on Apr 11, 2006 - 21 answers