Notes on discard reading
I. Large-scale patterns
A. Normal hand: honors and terminals < 28 < 3-7
Among floating tiles in the same class, those suffering overlap (such as ura-suji) are cut sooner.
The first number tile cut is rarely in a long suit.
Sotogawa.
B. Flush: off-suit < honors < flush suit
Overflowing compatible tiles, especially numbers, indicates tenpai or near tenpai.
C. Irregular: could indicate fast hand / chiitoi / chanta (rare) / kokushi
Suji less reliable for defense.
B+C: Flush, Irregular, and open hands are somewhat more likely to contain yakuhai.
Hands with flush chance may also keep and end up using off-suit dora instead.
Tedashi of incompatible tiles after compatible tiles is evidence against yaku (flush, tanyao, chanta).
Ryanmen chii with 23 or 78 after discarding 5 is evidence against tanyao, except when it's the first call (confirming tanyao from tanyao chance).
Riichi tile good for tanki indicates wait may be another good tanki, especially with chiitoi discard pattern.
Opponents discarding unsafe tiles in dangerous situations are likely tenpai.
II. Call reading
Most important hands to defend against are riichi, dora pon, flush, toitoi.
Among open hands, yakuhai is 2-3x as common as tanyao. Against open hand, pay attention to which yakuhai are live. Pon of 4/5/6 makes yakuhai slightly more likely.
On chii, discard nearby tile suggests area is complete (safe); discard non-nearby number suggests slimming (dangerous area).
Look at past discards, too. If the new meld was previously in a three-tile block, other three-tile blocks (perfect 1-away) likely.
Kuinobashi.
Tanki becomes slightly more likely if an opponent calls again after already appearing to be tenpai. Another tanki indicator is many tedashi after already appearing to be tenpai.
On a second call, particularly a chii, discard obvious safe tile makes yakuhai ankou unlikely.
Online, look out for pon lag, when it can be distinguished from chii lag. Useful for reading wall, blockade, and later slimming of the pon-lagged tile.
III. Gyakugiri
Gyakugiri is tedashi violating the usual order of honors and terminals < 28 < 3-7. It suggests slimming (or irime), which is otherwise hard to detect when tedashi follows usual order.
Gyakugiri 2 makes 1 more dangerous. If 5 was discarded previously, 1 and 4 are especially dangerous.
Gyakugiri 1 makes 2 more dangerous.
Dora followed by non-dora is also gyakugiri.
Above applies to recent gyakugiri. Gyakugiri may also indicate upgrade or slide.
IV. Joint dropping
Penchan dropping inside-out is weak evidence of near tenpai.
Kanchan dropping makes penchan unlikely unless dora- or yaku-related.
Kanchan dropping inside-out makes ryanmen or dora-/yaku-related wait likely.
3 followed by 1 unlikely to have any of 1234.
4 followed by 2 unlikely to have any of 2345; 11 possible.
2 followed by 4 unlikely to have 1.
Ryanmen dropping makes ryanmen especially likely (suji and honors safe). If riichi later, matagi-suji likely.
Staggered joint dropping (other tedashi in-between) implies remaining tiles related to later-cut tile. In case of staggered pair dropping, karagiri is possible, meaning a third identical tile may remain in hand. Apparent sequence dropping may also indicate karagiri.
Pair dropping makes tanki unlikely. Surplus-tile or perfect 1-away are likely.
Honor pair dropping makes ryanmen likely. Yakuhai pair dropping makes pinfu likely, and from followers of modern theory, tanyao likely.
Aka or dora pair dropping makes other dora pair likely (or big yaku).