How many pairs are there in a deck of cards?

In a standard deck of 52 playing cards, there are 13 ranks (Ace through King), and each rank has 4 suits (hearts, diamonds, clubs, and spades). A pair consists of 2 cards of the same rank, so there are 13 possible pairs in total, one for each rank. Therefore, there are 13 unique pairs in a standard deck of cards.