The correct answer to this open question is the following.
Although there are no options, we can say the following.
The practice of lining out was adopted by congregations in which a more or less musically literate leader sang one line of the psalm, which the congregation then repeated in unison.
This practice was a form of a capella song in which the first and main voice was sung by a leader who sang the tune and the lyrics so all the congregation could sing it. So the leader sang the part and people responded with the line. It was first used in old churches of Britain but the style has influenced modern-day songs.
