This article deals with the behavior of a planar joint taking surface defects into account. Two theoretical concepts are used: gap hull and difference surface. Gap hull represents the displacement space characterizing all possible relative displacements in a joint. Difference surface simplifies the study of contact between parts of that joint. A bijective relationship between these two concepts is developed and exploited. Two experimental methods are developed to validate these theoretical concepts. The first consists of measuring relative displacements between two parts of a planar joint using a measuring system designed specifically for this study. Results obtained from the first method are validated by comparing with the results of the second method in which the contacting surfaces of the planar joint are measured.