L sign (brain)
The L sign is one of the features useful in identifying the central sulcus on cross-sectional imaging.
It relies on identifying the superior frontal sulcus which intersects the precentral sulcus in an "L" 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 an "L" configuration, but rather in an "T"; this is then known as the upper T sign, to distinguish it from the lower T sign which is the confluence of the inferior frontal sulcus with the precentral sulcus 1.