A Japanese abacus is composed of beads or counters. The counters at the top deck are called 5-unit counters, while the counters at the bottom deck are called 1-unit counters.
The vertical bars holding the counters determine the place value of the numbers represented by the counters. From the right hand side to the left hand side are ones place, tens place, hundreds place, and so on. In some Japanese abacus, decimal numbers are included.
You can represent a number using an abacus by moving the 1-unit counters up the bottom deck and/or moving the 5-unit counters down the top deck. Moving the 1-unit up adds 1, 10, 100, etc. depending on the place value, and moving a 5-unit down increases the value of the number by 0.5, 5, 50, 500 and so on, depending on the place value.