Known as the first calculator, abacus evolved from set of stones placed in carvings, through tables with sliding parts, to different combinations of beads on wires, usually enclosed in a frame. It allows to add and subtract numbers easily, yet multiplication and division is possible by adding the same number or subtracting divisors. Today it's mostly used as maths teaching aid, but in former Soviet Union countries it's still used in many small shops because it doesn't need electricity and can be used very fast by experienced person.

There are different types of Abacus, a typical "Polish"
Abacus contains 10 wires, with 10 beads on each, in different colors
(each 5 beads). This abacus is used mostly as teaching aid. Abacus which
you can see in this page is so-called Russian Abacus ("Schyoty"), it is modified to
make typical prices computations easier. These modifications are:

1. A single wire with 4 beads positioned closer to the user, to compute
prices like "3.25" easier.

2. Middle 2 beads on each wire is different color.

3. Left bead of thousands and millions are different color.

4. Is is used vertically, like a book.

5. Usually it's "re-set" to zero by moving all beads to the right side,
not left.

Notice the last photo of different, smaller abacus, it has thousands marked red and has no additional third wire with 10 beads in the bottom row.

My abacus comes from... a cafe in Lviv, Ukraine, where it was used as a spare unit.