lining out
n.— «Though from central Scotland, the band have used Gaelic in their name (Dubh means black). This is a link to the claims of the similarities between Gaelic psalms and pre-blues gospel and spiritual music of black American culture. Various black congregations in the Deep South still worshipped with Gaelic words until the beginning of the last century—a practice known as “lining out.”» —“The blues cruise into Caol” Lochaber News (Fort William, Scotland) Mar. 22, 2007. (source: Double-Tongued Dictionary)
Further reading
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Not just black worshippers. From the 1950s I can remember white Southern Protestant congregations singing unfamiliar hymns (or singing more verses than we were used to). The leader would line them out, loudly speaking or almost chanting the next phrase just before we’d begin it. (The texts were in English, not Gaelic.) The practice did not seem odd at the time, but the effect must have been quite striking.
This practice is mentioned to in Chapter 12 of To Kill a Mockingbird, when Calpurnia takes Scout and Jem to church with her. It is referred to there as “Linin’.”
It is both described as it occurs and explained afterwards by Calpurnia.