# How to calculate the number of scatter plots?

if we have $p$ variables, the number of scatter plots we have is :

$p(p-1)/2$.

Why is this so? Is there any one who can explain this formula?

$$\binom{p}{2}=\frac{p(p-1)}2$$
As @NickCox stressed, the plot of $y$ versus $x$ contains the same points as that of $x$ versus $y$ (up to exchange of axes).