But this is not the only way of indicating successive differentiations. For,

if the original function be $y=f(x)$;

once differentiating gives $\dfrac{dy}{dx}=f'(x)$;

twice differentiating gives

$$
\frac{d\left(\dfrac{dy}{dx}\right)}{dx}=f''(x);
$$

and this is more conveniently written as $\dfrac{d^2y}{(dx)^2}$, or more usually $\dfrac{d^2y}{dx^2}$. Similarly, we may write as the result of thrice differentiating, $\dfrac{d^3y}{dx^3}=f'''(x)$.