Real-time Feynman rules at finite temperature

We reexamine the derivation of the real-time Feynman rules at finite temperature, starting with a time-contour with a finite initial time. By using a regularised free propagator which obeys the KMS condition, we are able to give a general derivation of the usual 1–2 formalism, provided we compute Green functions with at least one external line. The role of the vertical part of the contour is also examined.