Because of this, for every possible number of positions required, the OLTC would need a type variation. In case of compartment type OLTCs this will affect not only the selector design and the Geneva geometry, but also the terminal or selector boards. For practical and economical reasons the method of "run through" or "dummy" positions is introduced. This method allows to distribute the OLTC unused positions between the two end-positions of the tap selector. Electrically or mechanically, the tap selector can be made to run through these common contacts automatically