Keishiki tenpai 「形式聴牌」, or shaped tenpai, is a rule relating to answering the question "what constitutes a tenpai hand?" with quick certainty. As long as the hand is waiting for a tile that could exist anywhere outside a player's hand and calls, the hand is considered tenpai. This is a rule commonly announced by most organizations with their rulesets, covering a wide range of interest groups (pro leagues, jansous, overseas associations and clubs.
The hand cannot wait for a "fifth tile". A hand waiting for a fourth tile that is simply hidden, discarded, or used by someone else remains a valid tenpai in all circumstances. A hand containing a 1-2 penchan and a previously declared kan of 3-3-3-3 in the same suit, no more threes are available as winning tiles. Because of the prevalence of keishiki tenpai, it is assumed that if the rule is different, it would be announced what qualifies as tenpai or not. As a precaution, it is almost always mentioned.
- On Ron2, there has been an instance of a person being able to call riichi with a gutshot wait shape for a 6-pin when they have made a closed kan of 6-pin already. The hand was considered noten, but did not trigger a chombo penalty (mainly due to programming assuming no one could do something that could be viewed as faulty). This conforms to the usual keishiki tenpai interpretation that the hand was not in a valid tenpai shape when it came to scoring a drawn hand, but not okay for determining if a player was legally allowed to declare riichi.
- On Tenhou, there have been reports of a hand containing 12s44466688p with a kan of 3s, scored as in tenpai. This does not conform to the usual keishiki tenpai standard. Tenhou staff have confirmed that there was a decision to simplify interpreting if a hand was tenpai due to programming constraints. All hands that conform to keishiki tenpai are valid, as well as a few (such as the current case) that may not be.
Will have to do a complete rewrite. Noted on Reach Mahjong NY: "Having tenpai without yaku, for purposes of end of hand tenpai payments and so forth." So, content will be based on this.  KyuuAA (Talk:キュウ) 17:57, 5 May 2019 (UTC)
https://mahjongsoul.game.yo-star.com/?paipu=190508-4ebd32bc-71a5-4f4f-86a7-16066dfdc896_a925124703 KyuuAA (Talk:キュウ) 08:33, 8 May 2019 (UTC)
The article is confusing because it doesn't distinguish the different types of keishiki tenpai carefully enough.
(i) No yaku tenpai.
(ii) Tenpai with all winning tiles visible (dora indicators, discards, called tiles, closed kans by other players) and not in your hand. This isn't called keishiki tenpai, but karaten ("empty" tenpai). Whether it has a yaku or not is irrelevant, because you can't win with this anyway.
(iii) Hand is one tile short of four groups and a pair, but it needs a fifth copy of a tile in your hand to be completed. This is the second type of keishiki tenpai in the Arcturus wiki, which is much rarer and yet occupies the whole second section of the article. The Japanese Wikipedia article classifies this as being under karaten rather than keishiki tenpai.
Under typical rules, (iii) doesn't count as tenpai at all. Tenhou sort of does this, but it only looks at the part of your hand that isn't part of a chi/pon/kan. In other words, if you ankan a tile and end up having a kanchan/penchan wait for that tile, Tenhou will count this as tenpai; on the other hand, a shape like 2222345555999 would be no-ten, since it's waiting for 2 and 5 but all four copies of each are in your hand (and not part of a chi/pon/kan).