An acrostic psalm is one where the first letters of
consecutive verses (in Heb) make up a word, or the alphabet. The Heb alphabet
consists of 22 letters, which are listed as headings in Psa 119.
Psalms 9,10: In each the letter Waw is omitted and the letter
Pe is duplicated (in vv 16 and 22). Poss these two psalms were originally
complete alphabetical acrostics.
Psalms 37, 111, 112, 119: These are complete acrostic psalms
(the last being an 8-fold acrostic).
Psa 145: The letter Nun is missing in the Heb text. Many of
the ancient translations insert an additional v between 13 and 14 which begins
with this letter: "Faithful is the Lord in all his words and holy in all his
works" (RSV, NEB, LXX).
Other acrostics are to be found in Pro 31:10-31 and Lam
1,2,3,4 (chs 2,3,4 have 2 letters transposed).
Acrostics are an aid to memory, in an age when written copies
of the psalms (or any other Bible portions) were few, and when quite a number of
ordinary people were not able to read (Lam 30,31).