The presence of a hole, cut-out or void in a structure makes it difficult to be modelled for calculating natural frequencies. A theoretical basis for simplifying the modelling of cut-outs in a structure by attaching a negative structure is presented. The Dynamic Stiffness Method has been used to prove that this method yields the required natural frequencies. The derivations also show the presence of additional natural frequencies which correspond to the vibration of the positive and negative parts vibrating together while the actual structure with the hole or cut-out usually remains stationary.