With encouraging and hopeful lyrics, “Heartlines” comes right before “Spectrum”, making the duo a silver lining combo in the middle of a sorrowful album, as Florence Welch stated in a December 2011 interview with Elle:

I was thinking about positive messages on the record because I knew there would be kids listening. Like putting something in there that wasn’t just for me, like ‘Heartlines’ and ‘Spectrum’, they’re really earnest, saying don’t be afraid of who you are and follow your heart.

Sonically, the track brings the strongest drumming in the album, a sound that has been developing since “Drumming Song” from the previous album. In a track-by-track interview, Florence described “Heartlines” as an “almost tribal song”, in which she wanted the feeling of rolling forward, like a journey.

I wanted it to sound like something really epic. Like a gigantic battle in the sea unfolding, like mermaids versus pirates.