Nine of the biblical Psalms are acrostic poems. The general idea is a simple one: they comprise a sequence of uses of the Hebrew alphabet in alphabetical order. So, for example, in the two shortest complete acrostics, psalms 111 and 112, half a verse or less of material starts with successive letters of the Hebrew alphabet, aleph through tav in turn. In psalms 25 and 34 there is one verse beginning with each of the 22 Hebrew letters in turn. In psalm 119 there are famously 8 verses beginning with each of the Hebrew letters, giving the 176 verses that make this by far the longest of the canonical psalms.
These nine (or eight, see below) psalms are not alone in Scripture. Proverbs 31:10-31, the book of Lamentations and Nahum 1:2-8, all use the same acrostic device.
The acrostic psalms are not of a specific genre (or Gattungen). They can variously have strong elements of thanksgiving (34, 111), wisdom motifs and style (34, 37, 112), they can praise (9, 145), they can lament (10, 25, 119?) or there can be a focus on Torah (119). So what is the point to this literary device, one that in many cultures appears as something of a poetic oddity?
There are probably a number of interrelated ideas going on.
1. There is something here about convention and order. The biblical Psalms obey a number of conventions like all poetry rooted in a linguistic tradition. The acrostic form is one additional convention which gives a further constraint for the poet to work with. One of the challenges of poetry is to achieve something special within a set of rules that provide a framework. The idea of order takes on an especially poignant dynamic in the acrostic poems found in Lamentations 1-4 because of the horrors depicted there. The ordered response to the disorders experienced by Israel is the poets attempt to both highlight the dire nature of events and at least in language terms a step towards order, at least in defining the problem.
2. There is also a sense of completeness, just as in English we think of an A to Z as a compendium on a subject. A particular poem has in this sense made a complete point, argument or appeal. If this is so, then psalm 119 takes on almost manic proportions of completeness. Psalm 119 actually goes further still with its use of eight synonyms for God’s word or instruction. In nearly every verse one of these eight words appears.
3. Convention and completeness of this type also support the memorisation of the Psalms and this seems to have been important to the psalmist as Gordon Wenham has argued in a couple of recent books (Psalms as Torah, The Psalter Reclaimed).
A couple of final points are worth mentioning. Psalms 9 and 10 are not individual acrostics but they actually form a single acrostic. The fact that they belong together is further indicated in their being a single psalm in the LXX and in psalm 10 having no heading in the main Hebrew manuscripts. What is puzzling is why they have been separated and why the perfection of the acrostic has been lost in a couple of places by editing. There is also some disruption of the acrostic pattern in a couple of other cases. For example, in psalm 145 there is no clause corresponding to the ‘letter N’. Perhaps this is a deliberate disruption of perfection to make a theological point about the impossibility of perfection before the eschaton. As with many other aspects of the Psalter we can learn much, but also find some mysteries. It probably shouldn’t be any other way!