Sentence:  A 1996 survey showed that men under age 20 buy three times as many trifolds as <WSD>bifolds</WSD>. So a trifold wallet becomes a habit, even after it has expanded to the size and shape of a jumbo bar of soap. 
Meaning of the word marked with <WSD> tags:  A wallet, billfold, or carrying case with a single fold, so that it opens like a book.