Truss Rod Cover
A simple access plate
Truss Rod Cover
Gibson was the first company to use an adjustable truss rod in the necks of their instruments, starting in 1922. Their first truss rod cover had a sort of bell shape, just like this one:
Recently, Gibson has been enforcing their copyright of this shape, so it's not so common on other brands.
Most makers use their own designs, and some are quite attractive:
Here, Taylor's truss rod cover is made of the same kind of wood as the peghead veneer and it blends nicely.
The truss rod cover serves only a cosmetic function, and some makers, such as Fender, omit the cover entirely.