Given any natural number, you can add 1 to that number and get the next number you get is its successor.

The successor of 15 is 15 + 1 = 16, that of 18 is 18 +1 = 19 and so on.

The number 16 comes before 17, we say that the predecessor of 17 is 17–1=16; the predecessor of 20 is 20 – 1 = 19.