The influence of in-plane displacement and strain components on slope (first-order derivative of out-of-plane displacement component) fringe distributions in double-aperture speckle wedge-shearing interferometry is discussed in detail. The research results show that only the in-plane displacement component parallel to the centre line of double apertures has an influence on the slope fringe distributions. It is also shown that the in-plane strain components have no influence on the slope fringe distributions when utilising normal illumination and an axisymmetric system. A theoretical analysis and an experimental demonstration are given. The experimental results are in good agreement with the quantitative analysis.