h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

There was a time when improvisation was a standard skill. Hummel was said to be playing an organ concert in a town, when the bell of a nearby church began to ring. Hummel segued into the same key and rhythm, and made it sound planned.

The two girls in the picture might know the music better than anyone in the audience, so if the take liberties, who's going to know? But they'd better not agree to play something familiar unless they know it well, or can embellish like champions