Pon is a call that claims the tile an opponent has just discarded in order to complete an open triplet (minko). You must already hold the two matching tiles in hand: you reveal the pair, add the called tile and place the set face-up in front of you. This meld can never become concealed again, but a fourth self-drawn tile may later promote it to kakan.
Pon works on any opponent's discard. Under Tenhou rules, pon and daiminkan take priority over chii when several calls target the same tile. WRC 2025 gives priority to the first clearly announced group call; when announcements are simultaneous, pon or daiminkan takes priority over chii. After calling, you take the turn out of sequence and must discard. A pon can later become shouminkan with the fourth tile.
Declaring pon opens your hand, which forfeits riichi, menzen tsumo and pinfu, and it leaks information by exposing part of your holdings. The call speeds up your shanten and supports open-hand yaku such as yakuhai or toitoi, but a hand with no reachable yaku becomes impossible to win. Before every pon you must therefore make sure a valid yaku will remain, otherwise the points stay out of reach.