The traditional English carol "The Twelve Days of Christmas" has long been a favorite during the holiday season, standing the test of time through the centuries and inspiring amusing parodies like Bob Rivers' "Twelve Pains of Christmas."
But the song – which counts up from Christmas to Epiphany on Jan. 6 – has also become the peg for a whimsical way to gauge year-over-year inflation. Pennsylvania-based PNC Bank and other financial institutions have tracked the costs of each set of gifts...