gfortran implements a number of extensions over standard Fortran. This chapter contains information on their syntax and meaning. There are currently two categories of gfortran extensions, those that provide functionality beyond that provided by any standard, and those that are supported by gfortran purely for backward compatibility with legacy compilers. By default, -std=gnu allows the compiler to accept both types of extensions, but to warn about the use of the latter. Specifying either -std=f95 or -std=f2003 disables both types of extensions, and -std=legacy allows both without warning.