An identity function is a function whose output is equal to its input across its entire domain. The identity function can be represented by the following formula. For every $x\text{-}$value, the corresponding $y\text{-}$value is equal to that $x\text{-}$value, so the points on the graph of $f$ are in the form $(x,x)$ for all real numbers $x.$ This results in a diagonal straight line passing through the origin. The identity function is a building block for all linear functions. Any linear function has the form $f(x)=mx+b,$ where $m$ is the slope of the line and $b$ is the $y\text{-}$intercept. Then the identity function $f(x)=x$ is the simplest linear function with $m=1$ and $b=0.$