This paper analyzes leaky modes in open microstrip lines. The different possibilities and implications of the choice of the inversion contour associated with the spectral domain analysis are investigated. Specifically, we have focused on the leaky solutions obtained when the inversion contour entirely lies either on the proper sheet or on the so-called outgoing sheet. Solutions computed on the proper sheet can be considered surface-wave leaky modes and those computed on the outgoing sheet volume-wave leaky modes. The continuity between the different bound and surface-wave/volume-wave leaky solutions has been studied. A new kind of spectral-gap effect has been found as a consequence of such study.