In doing basic math, you work with many different groups of numbers. The more you know about these groups, the easier they are to understand and work with.
- Natural or counting numbers: 1, 2, 3, 4, …
- Whole numbers: 0, 1, 2, 3, 4, …
- Integers: … –3, –2, –1, 0, 1, 2, 3, …
- Negative integers: … –3, –2, –1
- Positive integers: 1, 2, 3, … (the natural numbers)
Note: Zero is neither positive nor negative. It is neutral.
- Odd numbers; integers not divisible by 2:
- … – 5, – 3, – 1, 1, 3, 5, …
- Even numbers: Integers divisible by 2:
- … – 6, – 4, – 2, 2, 4, 6, …
- Rational numbers: Fractions, such as
or 
. All integers are rational numbers; for example, the number 5 may be written as 
. All rational numbers can be written as fractions 
, with a being an integer and b being a natural number. Both terminating decimals (such as 0.5) and repeating decimals (such as 0.333 … ) are also rational numbers because they can be written as fractions in this form.
- Irrational numbers: Numbers that cannot be written as fractions
, with abeing an integer and b being a natural number. 
(the Greek letter pi) are examples of irrational numbers.