  This paper defines the mathematical convention adopted to describe an
electromagnetic wave and its polarisation state, as implemented in the PSRCHIVE
software and represented in the PSRFITS definition. Contrast is made between
the convention that has been widely accepted by pulsar astronomers and the
IAU/IEEE definitions of the Stokes parameters. The former is adopted as the
PSR/IEEE convention, and a set of useful parameters are presented for
describing the differences between the PSR/IEEE standard and the conventions
(either implicit or explicit) that form part of the design of observatory
instrumentation. To aid in the empirical determination of instrumental
convention parameters, well-calibrated average polarisation profiles of PSR
J0304+1932 and PSR J0742-2822 are presented at radio wavelengths of
approximately 10, 20, and 40 cm.
