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/GNU Fortran 7

6.1.8 Q exponent-letter

GNU Fortran accepts real literal constants with an exponent-letter of Q, for example, 1.23Q45. The constant is interpreted as a REAL(16) entity on targets that support this type. If the target does not support REAL(16) but has a REAL(10) type, then the real-literal-constant will be interpreted as a REAL(10) entity. In the absence of REAL(16) and REAL(10), an error will occur.

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https://gcc.gnu.org/onlinedocs/gcc-7.1.0/gfortran/Q-exponent_002dletter.html