"Mercy" is a soul-pop song by Welsh singer-songwriter Duffy. The song was written by Duffy and Steve Booker. It was released as the second single from her debut album "Rockferry" in 2008. The original key of the song is D major. "Mercy" became a commercial success, reaching the top of the charts in multiple countries and earning critical acclaim. The song showcases Duffy's powerful and emotive vocals, accompanied by a catchy and infectious melody. Fun fact: "Mercy" was featured in the popular TV series "Grey's Anatomy" and "Gossip Girl."