NEMA ( National Electrical Manufacturing Association )

This is a standard of measurments that relates to many items.
You may see in reference to a servos or steppers "NEMA 23" or "NEMA 34" etc.

This is the size of the fixing plate for these motors.

Divide the NEMA number by 10 to get the approx size of flange. (23/10 = 2.3 square)

(Inches)
NEMA Flange-Size / Bolt-Spacing / Hole-Dia.
23 / 2.22 / 1.87 / 0.18
34 / 3.41 / 2.76 / 0.21
42 / 4.22 / 3.53 / 0.27

More reference here....

http://sanyo-denki-online.com/g/23inoutw.gif
http://sanyo-denki-online.com/g/34inoutw.gif
http://sanyo-denki-online.com/g/42inoutw.gif