Though it hasn't been long since he and Stiles got together, Derek has still learnt the exact beating of Stiles' heart. Maybe his rapid learning curve also has something to do with the fact that he's been listening to Stiles' heart for as long as he can remember, almost ever since he came across him and Scott trespassing. Even at that first meeting, there was something, something he still can't pinpoint exactly, that attracted him to the tall and gangly teenager, his face dotted by the most perfect freckles and moles.

And after that encounter, Derek found himself listening carefully for Stiles' heartbeat, learning each beat, each spike and each fall. Soon he could pick it out from amongst Scott's unhurried beat; Allison's dancing and quick beat and Lydia's slow, steady thrum. Stiles' was unique. He missed a couple, would make up for it by a couple in quick succession, slow down and speed up, all in a matter of minutes. Listening to his heart beat was like riding a roller coaster.

It was listening to the steady but slightly panicked beat that kept him conscious in the pool as Stiles held him up while he was paralyzed. It was a sharp spike the next time Stiles saw him. Soon it began to spike whenever Derek appeared and would slow down considerably when he went. Derek smiled at that.

When the nogitsune took over Stiles' body, he also took over Stiles' heartbeat. At first Derek panicked when he could no longer recognize it, no longer pick it out from amongst the pack's. The nogitsune slowed it down to a darkly confident pound, no longer familiar. When it returned, Derek breathed a sigh of relief.

When Derek first kissed him, Stiles' heartbeat was so loud, so fast that he feared his heart might explode. When Stiles kissed back, all Derek could hear was his loud, but steadier this time, thud, thud in his ears.

There were new beats that he learnt when Stiles' started sleeping over. There was a very slow and solid one when Stiles fell asleep in his arms, which was ironic considering how many times he changed positions during the night, from being the little spoon, to tangling himself all around Derek like a vine, to hogging the covers, or even falling off the bed (once). There was a playful and ecstatic one when Derek took him out for dinner or when they wrestled under the covers, tickling each other. There was a soft, muted one when Stiles was calm, lying in his lap as he read.

But above every other type, Derek thinks he likes Stiles' heartbeat most when he wakes up next to him. The slow and steady pound of his sleep turns into a rush of rapid, little beats as he wakes and realizes where he is then slows back down to a confident pace as Derek leans over to kiss him, feeling their hearts beat in harmony.