Abacus

The earliest calculating device was known

as Abacus. Its exact origin is unknown. It is

invented 2000 years ago. Its has 2 section:

Heaven and Earth

which are separated by beam. The heaven is

also called the upper deck, and the earth is also

called the lower deck. The Abacus was used to

perform addition and subtraction. It is still used

today in china and some other countries.