The real number system evolved over time by expanding the notion of what we mean by the word “number.” At first, “number” meant something you could count, like how many sheep a farmer owns. These are called the natural numbers, or sometimes the counting numbers.
Natural Numbers (N)
or “Counting Numbers”
1, 2, 3, 4, 5, . . .
- The use of three dots at the end of the list is a common mathematical notation to indicate that the list keeps going forever.
At some point, the idea of “zero” came to be considered as a number. If the farmer does not have any sheep, then the number of sheep that the farmer owns is zero. We call the set of natural numbers plus the number zero the whole numbers.
Whole Numbers
Natural Numbers together with “zero”
0, 1, 2, 3, 4, 5, . . .
Even more abstract than zero is the idea of negative numbers. If, in addition to not having any sheep, the farmer owes someone 3 sheep, you could say that the number of sheep that the farmer owns is negative 3. It took longer for the idea of negative numbers to be accepted, but eventually they came to be seen as something we could call “numbers.” The expanded set of numbers that we get by including negative versions of the counting numbers is called the integers.
Integers (Z)
Whole numbers plus negatives
. . . –4, –3, –2, –1, 0, 1, 2, 3, 4, . . .
The next generalization that we can make is to include the idea of fractions. While it is unlikely that a farmer owns a fractional number of sheep, many other things in real life are measured in fractions, like a half-cup of sugar. If we add fractions to the set of integers, we get the set of rational numbers.
Rational Numbers (Q)
All numbers of the form 
, where a and b are integers (but b cannot be zero)
, where a and b are integers (but b cannot be zero)Rational numbers include what we usually call fractions
All numbers of the form , where a and b are integers (but b cannot be zero)
, where a and b are integers (but b cannot be zero)Rational numbers include what we usually call fractions
- Notice that the word “rational” contains the word “ratio,” which should remind you of fractions.
Irrational Numbers (I)
- Cannot be expressed as a ratio of integers.
- As decimals they never repeat or terminate (rationals always do one or the other)
