According to a tradition preserved in the Mishnah (Tamid vii, 3), the daily psalm of the Second Temple was sung in three parts, and at the conclusion of each pan, there was a pause. During this pause in the singing, two of the priests blew with their trumpets, and at this signal the worshippers prostrated themselves in prayer. A search throughout the Psalter for possible traces of this tripartite division in the Psalms generally reveals a close connection with the incidence of the word "selah". The occurrences of this word conform to this rule, namely, that wherever the word occurs, the psalm is in three sections, and the word occurs at the end of a section. Sometimes the break in the psalm is a natural one; sometimes it has been forced on the psalm. This latter occurs in cases where it was impossible to find a convenient place where a break was due from the choral point of view. There are never more than three selahs in one psalm, unless the psalm, as it is found in our texts, is composite. In such cases, the psalms, which are now joined together, separately conform to the above-mentioned rule.
Source: Norman Snaith, "Selah," Vetus Testamentum 2.1 (Jan 1952: 43-56 (43).
DOI: https://doi.org/10.2307/1515695
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