As a collector, every now and then you may have a need to raid your stock of duplicates for postage. But when you open the album you find a lot of non-denominated stamps. So how to figure their value and apply the proper postage to your envelope becomes a burning question.
Your friends at the USPS have solved the problem by creating a handy guide that gives the values of no-value stamps from the 1975 Christmas stamp (10 cents) right up to the current Liberty Bell design forever stamp (44 cents and certainly soon to go beyond).
There's even a handy PDF version, so you can print out the guide for ready reference.