The viola is a real-world instrument that is often a focal object in Adventure Time. It is typically played by Jake, however other characters also play it.
The viola is a member of the violin family. It is the middle voice of the group, having a lower pitch than the violin, but higher than the cello. It is often mistaken for a violin, due to both instrument's pitch and similarity in size and shape. The four strings from lowest to highest are C, G, D, and A.
In "Evicted!," Jake travels with his viola and bow while house-hunting, after Marceline seizes the Tree Fort from him and Finn. Given that it is the only thing he carries, it appears to be important to him.
In "Gut Grinder," the episode begins with Finn trying to look for Jake and stumbling across Jake's viola. After Finn plays it badly, Jake appears and takes his viola back, proclaiming Finn is horrible at playing viola. Later, when Jake is convinced that he is the Gut Grinder, Finn remembers Jake's viola and plays it badly once again, bringing Jake back to his senses.
In "The Eyes," Jake tries to lure the Horse away by playing Beethoven's "Moonlight Sonata" on his viola, an appropriate song given the scene's night time setting. This fails in coaxing the Horse, though Jake's rendition of Mozart's "Eine Kleine Nachtmusik" does succeed in attracting several snakes.
The back of Jake's viola reads toots. This is first seen in the episode "Gut Grinder," when Finn's backpack is cut open and Jake's viola falls out, as well as in "What Was Missing" while Jake is turned toward the camera.
Lady Rainicorn also plays the viola. In "My Two Favorite People," it is confirmed that Jake meets Lady every day at 4 o'clock on the dot so they can play the viola together.
In "What Was Missing," Jake breaks his viola, and in later episodes uses a taped-together viola.
In "King Worm," Jake plays his taped viola for dream Lady. This produces a sour note which turns into a mini King Worm.
Shelby, a worm befriended by Jake, lives inside of his viola as shown in "My Two Favorite People," "Video Makers," and "Holly Jolly Secrets Part II." Shelby also makes a cameo in "What Was Missing," when Jake smashes his viola, and in "Five More Short Graybles" where he unintentionally offends Tree Trunks.
The word "toots" is carved into the back of the viola.
Jake has been seen stroking the bow below the bridge of the viola. Doing this produces highly unpleasant noise in real life.