http://www.idpa.com/Documents/IDPARuleBook2005.pdf
Ppg 38-39
Moon clip carrier must:
A. Be designed for concealed carry and suitable for all day
continuous wear.
B. Be worn on a standard belt of no more than 1 ¾" width that
must pass through the belt loops on the shooter's pants.
C. Hold the moon clip with enough tension to allow it to be
turned upside down and retain it while fully loaded.
D.
Either be of open top configuration that covers at least 60%
of the diameter and100% of the cartridge height and retains the
moon clip by tension and a stud that protrudes through the
center of the clip,
or retain the moon clip with a snap or Velcro
closure.
Note: Moon clips may not be retained in the carrier by three
(3) or less cartridges. Side cuts are permitted for thumb and
index finger access only.