Originally Posted by John St.Amant

I call the one piece "bridge" and the 2 piece "split".
The lettering is based on a number line system . every sequential letter represents another 0.5 deg increment. The split blocks can be either X or not the X. The #1 represents a roll center change and does not effect toe in.
Narrow...
XD
XC
XB
XA
X
A
B
C
D
E
F
Wide
once again , every step is a half degree.
Hope this is a help to you .