The simplest type of function that we usually meet is called a linear function. It is a function given by the rule $y = ax + b$, where $x\in R$. The letters $x$ and $y$ that will appear in the formulations of all the functions that we will encounter are called variables.

The letters $a$ and $b$ are called coefficients. Coefficients, unlike variables, occur only in the general form of the function. They will usually be substituted by specific numbers. These coefficients affect the appearance of the graph of the linear function and thereby also the properties of this function. Whatever these $a$ and $b$ values are equal to, all the linear function graphs will have some features in common. In the following steps we will look at these features.