What characterizes a direct proportion?

A direct proportion characterizes a relationship between two quantities where the ratio of the two variables remains constant. This means that as one variable increases or decreases, the other variable does so at a constant rate. The graphical representation of this relationship is a straight line that passes through the origin, confirming that if one quantity is zero, the other is also zero. This linear graph illustrates that for every increment in one variable, there is a corresponding increment in the other, preserving the constant ratio. Such characteristics are fundamental in understanding direct proportion, making it clear why a function whose graph is linear is the correct characteristic of a direct proportion.