Washington, D.C.-area Swifties got a very special treat at Taylor Swift's concert Monday night (July 13) when the singer brought out pal Lorde to sing Lorde's huge breakout hit "Royals."

Swift is known for bringing out special guests during her concerts. Just this past weekend, she sang with The Weeknd and Nick Jonas for her two New Jersey dates. 

Watch the performance below.

After the duet, the two friends gushed on social media together. 

When your friend flies across the world to surprise your crowd with 'Royals' bc she's a wondrous angel. @lordemusic pic.twitter.com/zDdjtc7U3v

— Taylor Swift (@taylorswift13) July 14, 2015

but seriously, @taylorswift13 is the leader of a great and beautiful galaxy that i'd live in forever

— Lorde (@lordemusic) July 14, 2015

just laughing like "this is our job how lucky are we/this stadium is so loud and pretty/we are best buds/KILLING IT" pic.twitter.com/wNXNkLapFV

— Lorde (@lordemusic) July 14, 2015

This article originally appeared on Billboard.com.