The Anisotropic Layer Theory (ALT) has been extensively used for electric machine analysis, due to its ability to take finite iron permeability into account globally. However, the effect of the slot openings, which are often present in slotted electrical machines, is neglected. In this work, an extension of the ALT to account for slot openings is proposed. The classical ALT is combined with the Harmonic Modeling Technique (HMT) for slotted structures by assuming infinitely permeable tooth tips. A semi-analytical solution is described for the classical and the extended ALT model. The results of the semi-analytical models are compared to Finite Element Analysis (FEA) results.