The event record is the central repository for information about the particles produced in the current event: flavours, momenta, event history, and production vertices. It plays a very central rôle: without a proper understanding of what the record is and how information is stored, it is meaningless to try to use PYTHIA. The record is stored in the common block PYJETS. Almost all the routines that the user calls can be viewed as performing some action on the record: fill a new event, let partons fragment or particles decay, boost it, list it, find clusters, etc.
In this section we will first describe the KF flavour code, subsequently the PYJETS common block, and then give a few comments about the rôle of the event record in the programs.
To ease the interfacing of different event generators, a HEPEVT standard common-block structure for the event record has been agreed on. For historical reasons the standard common blocks are not directly used in PYTHIA, but a conversion routine comes with the program, and is described at the end of this section.