deriv LSK ETT STT aSTA ALPH OLDHOMEPAGE NEWHOMEPAGE
The retroflexes or /mUrdhanya or S;Tu ष्टु are S ष् and the /Tu.
A S;Tu ष्टु is either the letter S ष् or one of the five /Tu letters.
So the /STu are —
S ष् T ट् Th ठ् D ड् Dh ढ् N ण्
The word retroflex is Latin for "(tongue) bending backwards". In Sanskrit you may call them mUrDhni- मूर्ढ्निॱ or /mUrdhanya "made with the top", if you like.