an abacus for assisting in teaching the place value , addition , and subtraction , comprises a least 2 substantially parallel bars ; at least 18 beads mounted and independently moveable along the axis of each of the substantially parallel bars ; and means for maintaining each of the substantially parallel bars in a substantially fixed position with respect to each other . preferably , the different groups of beads located on the various parallel bars are coded with some form of indicia to denote their respective place in the 3 - member sequence of the base 10 numbering system .