The problem of a thin variable-length cylindrical post in a rectangular waveguide is investigated by an approach which permits the current distribution on the post to be determined accurately. This in turn permits the reflection coefficient to be evaluated accurately. The case considered is that where the frequency and waveguide dimensions are such that only the TE10mode can propagate. Off-centred posts, little considered in previous studies, are specifically considered, as are the resonance characteristics of the post. An experimental study is also reported which demonstrates the accuracy of the theoretical predictions.