The paper deals with the electromagnetic properties of the configurations formed by cutting narrow strips out of an infinite plane conducting sheet. The electrostatic distributions about linear slots and about a pair of parallel linear slots are obtained, together with those about linear conducting strips and parallel linear strips replacing these slots. The TEM waves along all these structures are next calculated, and comparison of these leads to an equivalence theorem between slots and strips. It is shown that a slot configuration gives the same field as a fictitious magnetic conductor, coinciding with the slot and carrying a magnetic current. This conception is helpful in suggesting the form and clarifying the behaviour of slot configurations, a number of which are illustrated in the paper.A theorem of inversion is next derived, which is useful in calculating the effect of interconnected slot and wire structures.In the last section, these theorems are used to give an approximate solution for the radiation field of a half-wave slot and its radiation resistance seen from a shunt connection across its centre.