There are 124 legal 2-letter words in the SOWPODS official tournament word list. All of them are listed below, ordered from the most commonly played QI to the least commonly played MM (on ScrabbleGuru we call this ordering "playrank order"). Every one of these words gets played very often and any serious Scrabble player must memorize them all.
1. qi
2. re
3. in
4. er
5. it
6. en
7. et
8. ne
9. id
10. we
11. te
12. el
13. ed
14. ar
15. or
16. ti
17. za
18. li
19. if
20. de
21. gi
22. ja
23. ef
24. di
25. fe
26. at
27. an
28. ee
29. on
30. xi
31. ex
32. zo
33. jo
34. ag
35. aw
36. ad
37. al
38. ur
39. na
40. ta
41. no
42. io
43. ow
44. be
45. la
46. pe
47. un
48. to
49. da
50. od
51. wo
52. ut
53. lo
54. ea
55. go
56. bi
57. ax
58. oe
59. ae
60. em
61. fa
62. pi
63. he
64. ye
65. eh
66. ab
67. ug
68. ny
69. of
70. op
71. do
72. ai
73. ob
74. hi
75. me
76. ox
77. nu
78. oi
79. es
80. ki
81. is
82. mi
83. gu
84. st
85. si
86. pa
87. ba
88. up
89. am
90. ah
91. ya
92. ha
93. aa
94. ay
95. om
96. as
97. ka
98. bo
99. xu
100. ou
101. ch
102. oo
103. po
104. ma
105. yo
106. oh
107. oy
108. ko
109. ho
110. so
111. yu
112. os
113. mo
114. uh
115. um
116. fy
117. us
118. mu
119. by
120. sh
121. my
122. hm
123. ky
124. mm
This list is original research from ScrabbleGuru. Come visit us for more!