First of all we have a letter at the front of all of our products. This letter, or letters, refers to the style of decoder. Therefore, the M1 is the same style as the M3 and as the M4, meaning it is the same size and shape just with added functions. The T1 is also the same style as the T4X and the T6X. The only place that this doesn't apply is with the FL2 and the FL4 where FL simply stands for fleetlighter. Second there is a number behind the letters. This number stands for the number of light functions that are on the decoder, i.e. A4X has 4 functions, MC2 has 2 functions. The only exceptions to this are the T1 and the M1. These decoders have 2 functions but when we changed to this method of naming products these decoders were already very popular so we left them with the same name. Also, some n-scale drop-ins are simply named after the locomotives that they install into and the numbers are therefore not related to the functions. Third. Some of our decoders have an X on the end of them. This stands for our X series feature and we added it to decoders as they got the new features. However, at this point in time all of our decoders have the X series features but not all of them have the X. Again that was because we didn't want to change some of the names because they are so well known, and also some of our decoders came out after it was already well known that they all had the X series features.