PRT (CADKEY)
From Just Solve the File Format Problem
PRT is a format used by the CADKEY software from 1984 to 2004, during which time it was the most common variety of "PRT" file. A number of other programs also have different-format files under the PRT name, so things can get confusing. In 2004, CADKEY was replaced with KeyCreator, which uses the CKD format instead. KeyCreator can still read CADKEY PRT files, from version 5 and up.
