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A decimal number is composed of two parts separated by a comma: the whole part is the number to the left of the comma, and the fractional (or decimal) part is the number to the right of the comma.

The number 12.67 can be broken down as follows:

12.67 = 12 + 0.67

12 is the whole part, and 0.67 is the decimal part (or fractional part since it can be written as 67/100 - "sixty-seven hundredths"). The decimal part is always less than 1 because it is a fraction of the unit.

This animation allows you to visualize the value of a digit depending on its position in the number, as the scales are respected.

Scientific notation is a way to simplify the writing of very small (and very large) numbers (see the Kilo Mega Giga animation). It is often useful to approximate a number with a limited number of significant digits:

- Ex1: 1.243078 can be approximated to 1.24 with only three significant digits.
- Ex2: a length of 0.00000178 m can be rounded to 0.0000018 m with two significant digits, then written as 1,8x10
^{-6}m in scientific notation (1.8 µm).

*Note: The proportions are respected in the illustrations.*

- To visualize small numbers up to billionths.
- To estimate small numbers and find an approximate value.
- To understand the concept of "significant digit."
- To understand the meanings of prefixes DECI, CENTI, MILLI, ... MICRO ... NANO.
- To write decimal numbers using scientific notation.

The decimal positional system is now the most widely used in the world, but it should be noted that other **numbering systems** have existed, such as the base 5 (based on the five fingers of a hand),…