Within CalculiX, Guido is responsable for the core finite element program. Having grown up at a time when C was born and FORTRAN the standard for engineering calculations, the core of CalculiX is written in a mix of C and FORTRAN. The first lines of code were written in the autumn of 1998, and were heavily influenced by the FEAP program by Zienkiewicz and Taylor. At first intended as a simple finite element program for linear elastic static calculations with 20-node brick elements using reduced integration, the project gained momentum of its own: frequency and buckling calculations were added, nonlinear geometric features were implemented, and nonlinear material behavior was started at the end of 1999. Subsequent features were cyclic symmetry, shells and beams, heat transfer calculations, steady state dynamics, one-dimensional networks and computational fluid dynamics.