The upper T sign is one of the features useful in identifying the central sulcus of the cerebral cortex on cross-sectional imaging.
It relies on identifying the superior frontal sulcus which intersects the precentral sulcus in a "T" junction, thus defining the precentral gyrus. The central sulcus is the next posterior sulcus 1. Occasionally the junction between superior frontal sulcus and precentral sulcus is not in a "T" configuration, but rather in an "L"; this is then known as the L sign 1.
It should be distinguished from the lower T sign which is the confluence of the inferior frontal sulcus with the precentral sulcus.