Apex (diacritic)
Although not really known by some modern Latinists, the sign was actually used a lot during classical and postclassical times. The reason why it is not usually noticed is probably because of its smallish size and the fact that it is usually quite thin. People who look carefully will soon start to notice apices in the exhibits of any museum, not only in many of the more formal  epigraphic writings, but also in handwritten  palaeographic things.