A number line is a line with integers (simply ordinary numbers)(...-3, -2, -1, 0, 1, 2, 3...) on it extending forever in both directions. Usually, zero (0) is placed in the middle of the line. The numbers have an equal space from each other. Other numbers in between the numbers on the line represent rational, irrational, and other types of real numbers.
There is another number line, equally valid and useful, that extends vertically through 0 (the origin) at right angles to our traditional number line and uses imaginary numbers (i). These so called imaginary numbers extend above and below the traditional number line and are used in very real calculations in many areas of science and technology and also extend in both directions forever (into infinity).