Min Wall vs Nominal Wall

All,

(12x10) reducing tee is defined as 1.225" Min Wall x 1.250"Nominal Wall.

Since we can procure only with Nominal Wall thickness so we must specify Nom wall at both ends.

Pipe of 12" is denoted by MW & 10" by nothing. Pl. explain.

Also let me know how to calculate NW from MW.

Thanks