PYTHIA was originally written in FORTRAN 77, until the 2007 release of PYTHIA 8.1 which was rewritten in C++. Both the Fortran and C++ versions were maintained until 2012 because not all components had been merged into the 8.1 version.
PYTHIA was initially written in FORTRAN 77, until the release of PYTHIA 8.1 in 2007, which was redeveloped in C++. Both the Fortran and C++ versions were supported until 2012 due to incomplete component integration into the 8.1 version.