Around about the 1118 series of saws, Stihl model numbers started to be roughly based on CI (cubic inches) of the engines. The 039 was 3.9 CI, the 044 was 4.31 CI. Then about 10 years ago they flipped the zero from the front to the end of the model numbers, and the 039 became the MS390, and the 044 became the MS440. MS was added as a prefix, and is an acronym for MotorSagen, which means 'chainsaw' in German. Chainsaws are commonly listed in CCs, but bars are still listed in inches. Its a mixed bag of English and Metric standards in chainsaws. As for standardizing, we here in the US are still using pounds, gallons, inches and miles. My 8th grade science teacher said we were going to change over 30 years ago, but it has yet to happen.