Like Last Christmas, it’s in D major and uses the trusty I-vi-ii-V progression, so - as various people have demonstrated - creating a mash-up of the two songs is pretty straightforward.

Taylor Swift ‘Eras Tour' Book Revelations: BTS Secrets and More "By the end of the tour, I was playing between four [and] five mashed-up songs in a night in the acoustic set," she recalled.