A Desmós, alternatively the soul, skein, or knot, is the bundle of strings that forms inside a living creature. It is also what binds a creature to the Istós, and is what gives living beings the ability to access that network to use the tuning arts. Each knot possesses a distinct number of strings, sometimes called heartstrings, and from the knot there extend smaller filaments to the greater expanse of the Istós, connecting living things to one another and the material world.
In most sentient creatures, there are seven heartstrings, each of a “color” perceivable by those trained in Tuning. As one goes down the ladder of the animal kingdom, creatures have fewer and fewer heartstrings, until one reaches plants, which rather have small bundles of filaments.