Final answer:
The congregation engages in lined-out hymn singing, where a deacon or elder sets the tune and the congregation joins in. This practice allows everyone to participate in singing hymns, even if they don't have hymnbooks or can't read.
Step-by-step explanation:
When Scout and Jem accompany Calpurnia to church, Scout is surprised that there are no hymnbooks. In the church, the congregation engages in a practice called lined-out hymn singing. A deacon or elder 'lines out' a couplet of the text in a sing-song voice, setting the tune for the hymn. Then, the deacon starts singing, and the congregation joins in on the second syllable. The congregation sings this way because many members may not have access to hymnbooks or may be illiterate, and lined-out hymn singing allows them to participate in the singing of hymns.