Improvising and adding verses to existing songs was once a strong part of oral tradition. In fact in some oral traditions prior to widespread literacy ballad singers, we are told, used to remake songs every time they sang them using well used formulae and the basic plot.

Of course shanty singing is a good example of this improvisation.

The ubiquitous tune and chorus of the 18thc was 'Derry Down', that Derry Down that is used for 'The Deadnaught' and 'King John and the Abbot of Canterbury' and many thousands of other texts.

Bawdy singarounds often produce improvised verses particularly with catalogue songs like 'Sing us another one do'(Limericks) and 'The Four and twenty virgins'.

In fact we used to run a bawdy late-night session at Whitby festival. One night Johnny Handle was stood at the bar listening in for a couple of hours without contributing but getting well sozzled. We were just about to end the proceedings when Johnny lurched into the middle and launched into a song which he must have been making up on the spot as each verse was about a different person in the room. Needless to say it was well within keeping with the night's theme.