Mahjong's winning condition fits in one sentence: 14 tiles that split exactly into 4 “melds” + 1 “pair” (4 × 3 + 2 = 14). A meld is either a chow or a triplet.
Chow: three consecutive tiles of one suit
For example 2-3-4 of Bamboo, or 7-8-9 of Characters. It must be one suit — a 2 of Characters, 3 of Bamboo and 4 of Circles is not a chow.
Triplet: three identical tiles
For example three 9s of Circles, or three East winds. Honors have no numbers, so they can only take part through triplets (or as the pair).
Pair: two identical tiles
The last two tiles form the “pair” (sometimes called the eyes). Any tile works — a pair of 5s of Characters, a pair of Whites, anything.
A complete example: these 14 tiles win
The hand below is two chows, two triplets and a pair — a standard winning hand:
To check a hand yourself: pin the obvious triplets first, then connect runs, and see whether what's left is one pair. And when your hand is unusually pair-heavy, remember the Seven Pairs exception — seven pairs (14 tiles) also wins, no melds required.
Common beginner mistakes
Mistake 1: wrong tile count. A win is always 14 tiles (4 × 3 + 2); with 13 you are merely “ready”, one tile short. Mistake 2: runs of honors. 東, 南 and 西 have no ordering — they can't connect like 1-2-3. Mistake 3: mixing suits. Runs must be one suit; consecutive numbers across different suits are meaningless.