The 15-88 can also be electric. The difference is whether the motor is directly attached to the upper plate on the back of the machine (15-91) or attached behind the machine with a belt drive (15-88). My mother's 15-88 (someday mine) was purchased new in 1935 as a treadle. Three months later Dad bought the Singer motor for it. Singer offered the 15-88 both with and without the electric motor, but both have the same treadle base. I have verified all this with the original manual, as well as my mother's personal story. I love Mom's/my 15-88 and know you will love yours.