A thinny is a weak spot in reality where the fabric of the worlds has been worn thin.
Thinnies have the appearance of large blobs of mercury, and emit a warbling sound similar to a musical saw that can set a person's teeth on edge and/or hypnotize a person. Besides the insanity-inducing buzzing and warbling sound, the thinny plays on a person's thoughts; telling them what they want to hear and promising a fine outcome, though death is ultimately more likely. Transportation into other universes is possible by simply walking into a thinny, but this is a rare outcome.
David Drayton and his son became trapped in a grocery store when a thinny created by the Arrowhead Project released a thick, extra-dimensional mist (possibly the Todash Darkness) populated by bizarre creatures on the town of Bridgton, Maine.
After escaping Lud aboard Blaine the Mono, Roland Deschain, Eddie Dean, Susannah Holmes, Jake Chambers and Oy travel through a thinny into a parallel world ravaged by the superflu. Hiking along an Interstate highway in Topeka, Kansas, Roland describes an instance in his childhood in which he encountered another thinny in the barony of Mejis.
Morgan Sloat is also capable of using a magic key that tears open a rift between Keystone Earth and the Territories which greatly resembles a thinny; Jack Sawyer describes the phenomenon as like looking through a "dirty stain glass window." However, while an unpleasant sound accompanies the tear, it is not a warbling but a sound described as a "loud ripping sound" that rises higher and higher without stopping. It is possible that there may be different 'types' of thinny and these discrepancies are simply examples of this.