Let a and b be two real numbers. (a, b) is called a pair of real numbers a, b. But (a, b) is called an order pair if (a, b) is different from (b, a). In the ordered pair (a, b), a is called the first entry or first coordinate and b is called the second entry or second coordinate. If two numbers are equal, say, a and a then (a, a) is also an ordered pair in which the first entry as well as the second entry is a.