How does An Abacus Work?

On an abacus each set of beads represents a place in the number line, like thousands, hundreds, tens, ones. When you add or subract you move the amount of beads in each place. Say you wanted to subract fifteen from twenty. You would set the beads at 20 and then move one from the tens place and five from the ones place and you would end up with five beads left in the ones place.