Definition:

Points on one

Points on the other

An arrow in the diagram has its tail on a point representing a selected number, $a$, on the source (domain) line.

The head of the arrow points to the function value, $f(a)$, for the number $a$, represented by a point on the target (co-domain) line.

Conventions: When the axes are displayed vertically

Both lines are oriented with the larger numbers higher on the axes.

Both lines are oriented with the larger numbers further to the right. In some uses the representation is reversed with the bottom line representing the source and the top line representing the target.

In this resource, mapping diagrams will be displayed vertically.