Keishiki tenpai 「形式聴牌」, or shaped tenpai, is otherwise known as a tenpai hand with no yaku. Some cases of tenpai also involve waiting for a so-called fifth tile, which is nonexistent. Strategically, players often push for late tenpai hands regardless of yaku state for ryuukyoku to gain small points rather than lose them. This term can be shortened to keiten for ease.
Players do not necessarily aim for keishiki tenpai until late into the hand. Usually, consideration for such action begins on the third row of discards. Ryuukyoku imposes point gains or losses. Even if minimal, players may find themselves motivated to be on the positive end of the exchange rather than the negative. Plus, tenpai at ryuukyoku also provides a chance for either haitei or houtei to outright win the hand. Finally, players opting for this push for tenpai must also be wary of players already in tenpai, making defense considerations to be noted. With that said, it is preferable to lose points via noten rather than dealing into larger hands.
Some house rules factor keishiki tenpai into ryuukyoku, where hands even in tenpai may not receive tenpai payment. Instead, the points for noten are deducted just because the hand lacks a yaku. However, this practice is generally uncommon.
- Keishiki tenpai in Japanese Wikipedia