Montage
The Threepenny Opera's style is all over the place. Sometimes the characters are singing songs that don't seem to have anything to do with the plot, like the Pirate Jenny song that Penny performs at her wedding party:
And a ship with eight sails and
All its fifty guns loaded
Has tied up at the quay. (1.2.319-321)
See what we mean? This isn't a pirate play; it's not even set at the beach. But the violence of the song reflects the play's violent themes.
Other times they're having a very realistic argument complete with off color phrases. Check out this one between the Peachums:
MRS. PEACHUM. A fine opinion of your daughter you have.
PEACHUM. The worst. The very worst. A lump of sensuality, that's what she is.
MRS. PEACHUM. If so, she didn't get it from you. (1.1.177-179)
And other times it's not even the characters that are communicating, but strange, abstract signs that seem to be scene titles but seem more like external commentary. Before Polly sings to her parents of her "marriage," the stage directions say, "Three lamps are lowered from above on a pole and the signs say: 'IN A LITTLE SONG POLLY GIVES HER PARENTS TO UNDERSTAND THAT SHE HAS MARRIED THE BANDIT MACHEATH:'" (1.3.11-14). These are all great examples of the modernist montage that Brecht was known for.