The US Army in WW2 used (and still uses) M-numbers for the designation of equipment in service, followed by the variant (E at that time) and the class of equipment. On Wikipedia there is a list of military vehicles by M-number: List of U.S. military vehicles by model number - Wikipedia I find it very confusing. It would seem that different classes of tanks, for instance light, medium, and heavy, and other fighting vehicles like armored cars and GMC received their M-number independently. For instance there is a M3 medium tank and a M3 light tank. However there seem to be inconsistencies and gaps in the numbers that I cannot explain. For instance, the next light tank after the M3 is the M5, followed by the M22 and M24. After the M4 medium tank there is no M5. The T7 (T was the Trial or Test designation) was accepted as M7 medium tank but only a dozen were made. The Ordnance Department asked the T23E3 and the T20E3 mediums to be standardized as the M27 and M27B1 (they were rejected). The first heavy tank was the T1E1 but it was standardized as M6 not as M1. I cannot see the logic and the method behind it. Do any of you ppl have any explanation for this? What were the criteria followed in these designations?