Also, THAC0 and BAB can be directly converted:

BAB 0 = THAC0 20
BAB 1 = THAC0 19

etc...

In (A)D&D all Clases started with THAC0 20, but in (O)D&D they all started with THAC0 19. This is reflected in the way that full BAB 3.x Base Classes start with equivalent THAC0 19, whilst partial BAB 3.x Base Classes start with equivalent THAC0 20. THAC0 is itsef a simplification of (A)D&D 1.x Combat Matrices, which work slightly differently.

(A)D&D AC* and D&D 3.x AC are also related:

AC* 10 = AC 10
AC* 9 = AC 11

etc...

Although, strictly speaking, -10 AC was the theoretical maximum, some Dragons had AC -12. Also, many magical effects did not lower AC, but imposed penalties on enemy Attack Rolls (which effectively could raise AC above -10/-12).