An abacus is an ancient counting tool consisting of a frame with rods on which BEADS are strung. The term 'abacus' is derived from the Greek word for 'calculating board'. The SUM of the abacus is three letters, ADDER is five letters, BEADS is five letters, BEAD is four letters, ADDS is four letters, ADD is three letters, ROD is three letters, and COUNTER is seven letters. The action of adding is represented by the word ADDED, which is five letters.