Double prime(a″) symbol in LaTeX

The easiest way to print a prime symbol in a latex document is to use a prime(') symbol or \prime command. Then you understand that for double prime you have to use the symbol or command twice at the same time.

\documentclass{article}
\newcommand{\dblprime}{^{\prime\prime}}
\begin{document}
   $$ f''(x) $$
   $$ f^{\prime\prime}=\frac{d^2 f}{dx^2} $$
   % Use \newcommand for create custom command
   $$ f\dblprime $$
   $$ f\dblprime_{xy}(x,y) $$
   $$ 18^\prime 36\dblprime 16 $$
   $$ 1^\prime 33\dblprime 02 $$
\end{document}

Output :

Different packages already have macros defined for this symbol. For example the stix package.

\documentclass{article}
\usepackage{stix}
\begin{document}
   $$ f^\dprime $$ 
   $$ f^\dprime(x) $$
   $$ f^\dprime=\frac{d^2 f}{dx^2} $$
\end{document}

Output :