The paper describes a new experimental testing procedure being developed by the authors for characterising the shear behaviour of mortar joints under combined shear and compression loading. The test apparatus subjects a single joint specimen of solid or annular circular cross section to normal compressive force combined with torsion. The choice of circular cross section results in predictable distributions of normal and shear stresses across the mortar joint, allowing shear behaviour at a material point to be characterised. The latter has been a widely reported shortcoming of existing mortar joint shear tests which all result, to varying degrees, in complex, non-uniform distributions of shear and normal stresses across the mortar joint. The paper describes the proposed test methodology in detail and provides a theoretical evaluation of the test. A soon to be conducted experimental program will be reported in another paper.