Numbering the Psalms in the Septuagint

This will be an informational post. When reading academic literature, it is common to see references to the Psalms given twice. For example, in his commentary on 1 Peter, Craig Keener cites "LXX Psalm 33:9 (ET Ps. 34:8)" (126). "LXX" here refers to the Septuagint, the Greek translation of the Hebrew scriptures, and "ET" refers to the English translation, derived directly from the Hebrew Masoretic text. So what is the difference between these two numbering systems?

Psalms 1-8 and 148-150 are numbered the same in each system. Psalms 9 and 10 are combined as Psalm 9 in the LXX. This conflation means that Psalms 10-112 in the LXX are each shifted forward one, covering ET Psalms 11-113. Thus, Psalm 33 in the LXX is Psalm 34 in the ET. At this point the divisions get more complicated. ET Psalms 114 and 115 are conflated in the LXX as Psalm 113. The first nine verses of ET Psalm 116 form LXX Psalm 114 and the final ten verses form LXX Psalm 115. ET Psalms 117-146 appear in the LXX as Psalms 116-145, resuming the familiar pattern of being one chapter ahead. The first eleven verses of ET Psalm 147 form LXX Psalm 146, and the final nine verses form LXX Psalm 147. The chapter numbers now aligned, Psalms 148-150 are the same.

These differences can be visualized in the following table:

ET Psalms 1-8 -> LXX Psalms 1-8
ET Psalms 9-10 -> LXX Psalm 9
ET Psalms 11-113 -> LXX Psalms 10-112
ET Psalms 114-115 -> LXX Psalms 113
ET Psalm 116:1-9 -> LXX Psalm 114
ET Psalm 116:10-19 -> LXX Psalm 115
ET Psalms 117-146 -> LXX Psalms 116-145
ET Psalm 147:1-11 -> LXX Psalm 146
ET Psalm 147:12-20 -> LXX Psalm 147
ET Psalms 148-150 -> LXX Psalms 148-150

There is scholarly debate as to which numerations are correct. For example, Psalms 9 and 10 appear to be a Hebrew acrostic poem that is best read as one continuous psalm. On the other hand, LXX Psalms 146-147 are often argued to be one poem, correctly captured in the ET as Psalm 147. One might also notice a difference in verse numbers. This is partially due to dispute over whether to count the title of the Psalm as a verse or rather to start the numbering with the actual text of the psalm. 

When citing the psalms, a single unlabelled reference will usually be taken to mean a reference to the English text. Usually, that will be sufficient, though more scholarly work may call for the double reference.