Pebbles (calculos in Latin) are the "bits" used in the Ancients' four function calculator / computer. The Ancient Computer's normal mode is to work with numbers in what we would call exponential notation. Decimal numbers can have up to 10 significant digits in the coefficient (a fraction < 1 with no leading zeros) and up to 4 significant digits in the exponent (a radix shift). Duodecimal and sexagesimal numbers can have up to 5 significant digits in the coefficient and up to 2 significant digits in the exponent. Coefficients and exponents can be either positive or negative. Built-in error checking is included since an addend can be entered and checked before accumulation. The Ancient Computer is time tested; it or its predecessors have been in use since before 2000 BC.