Formal methods model consists of a set of activities that establishes the formal mathematical specification of computer software. In this model initial a mathematical specification is constructed, which can be written in either languages VDM or languages Z. The specification is then transformed stage by stage into the working software. That why it is also called as the transformation mode. So the main aim of this model is to convert the formal mathematical specification of a system specification to an executable program. This model is concerned to the theoretical work on formal specification. This basic formal system development is shown in fig.2.16.