Why such tiny words matter so much
Two-letter words aren't about scoring 4 points for AT. They're about parallel plays: laying a word alongside an existing one so that every touching pair of tiles forms a valid two-letter word. One six-tile play can score for the main word plus five two-letter crosswords in the same turn — that's how experienced players routinely pull 30+ points from unglamorous racks. They're also your escape hatch: when the board is closed and your rack is ugly, knowing that ZA, QI and JO are valid turns dead turns into 20-plus points on a premium square.
The list, grouped for learning
Roughly a hundred two-letter words are valid in the ENABLE dictionary this site uses (tournament lists differ by a few entries). Don't memorize alphabetically — group by what they do:
The ones you already know: AN, AS, AT, BE, BY, DO, GO, HE, IF, IN, IS, IT, ME, MY, NO, OF, ON, OR, SO, TO, UP, US, WE — free confidence, nothing to learn.
The power tiles (learn these first): QI (life force in Chinese philosophy), ZA (slang for pizza), XI and XU (Greek letter; Vietnamese coin), JO (Scots for sweetheart), KA (Egyptian spirit). These six turn the highest-value tiles in the bag into easy plays — QI on a double-letter square is 21 points for two tiles.
Vowel dumps (for drowning racks): AA (rough lava), AE (one), AI (a sloth), OE (a whirlwind), OI, EAU is three letters — but you get the idea. Also: AB, AD, AG, AH, AL, AM, AR, AW, AX, AY.
The E-column: ED, EF, EH, EL, EM, EN, ER, ES, ET, EX — mostly names of letters (EF is "F", EM and EN are printers' measures). Learn them as a chant.
The odd ones that win arguments: BA (Egyptian soul), BO (a friend), DE (of, from), FA and TI (music notes, along with DO, RE, MI, LA, SO), HM and MM (yes, really), LI (Chinese distance unit), MU, NU, PI (Greek letters), NA, NE, OD, OM, OP, OS, OW, OX, OY, PA, PE, SH, SI, TA, UH, UM, UN, UT, WO, YA, YE, YO.
How to actually memorize them
Don't study the list; play it. Three approaches that work: (1) Chunk by letter — each day, own one starting letter ("A day": AA AB AD AE AG AH AI AL AM AN AR AS AT AW AX AY). (2) Learn the stories — words with meanings stick: a sloth (AI), lava (AA), pizza (ZA), a sweetheart (JO). (3) Drill with the tools — enter any two tiles into the Scrabble finder and see what's valid; wrong guesses you check are remembered far better than lists you skim.
Using them at the board
Once the list is in your head, change how you look at the board: every existing word's edge is now a scoring surface. Before playing the "obvious" spot, check whether the same word laid parallel to a neighbor scores double or triple by forming two-letter crosses. And defensively: when you know the two-letter words, you can see which openings you're leaving your opponent — sometimes the best play is the one that leaves no vowel adjacent to a triple-letter square, because you know exactly what QI would do there.
Related: Q without U words for the deep dive on the trickiest tile, and the Scrabble word finder to check any rack. WWF players: most of this list is valid there too, with a few differences — the in-game test-play is the referee.