The SG2 file extension is associated with the Band-in-a-Box. File contains saved song. BiaB uses variations of .sg* and .mg* .sg* stands for a song without a melody. The last digit (1 through u) stands for the style used in that song. You will find the built-in styles (usually starting with zz) have numbers or lower letters after the .sg -- so if the song without melody was saved using the zzjazz style, the extension would be .sg1, the built-in country 12/8 = .sg2. The only one that is different is the .sgu extension. The "u" stands for a user style. Most of the styles are user styles (BiaB originally came with only 24 styles, all the rest are what BiaB calls user styles). .mg* are just like .sg* songs except for the fact that they have a melody. If you enter a melody in a .sg5 song and then save it, you will find a .mg5 song gets saved.