The effects of base width modulation (‘Early effect’) on the d.c. and low-frequency incremental output characteristics of a bipolar junction transistor are reviewed critically, using only a basic consideration of the physical electronics of device operation. Attention is focused on the dependence on collector-base voltage of the majority carrier charge in the base of a device having an arbitrary base doping distribution. The conditions under which a single voltage parameter (‘Early voltage’) can be used to describe the output characteristics, independent of baseemitter drive conditions, are examined and some implications of the results to the circuit engineer outlined.