A further development of the waterfall model led to the so called "V-Model". If you look at it closely the individual steps of the process are almost the same as in the waterfall model. Therefore I will not describe the individual steps again, because the description of the waterfall steps are suitable for the V-Model as well. However, there is one big difference. Instead of going down the waterfall in a linear way the process steps are bent upwards at the coding phase, to form the typical V shape. The reason for this is that for each of the design phases it was found that there is a counterpart in the testing phases which correlate to each other.
The time in which the V-model evolved was also the time in which software testing techniques were defined and various kinds of testing were clearly separated from each other. This new empasis on software testing (of course along with improvements and new techniques in requirements engineering and design) led to the evolution of the waterfall model into the V-model. The tests are derived directly from their design or requirements counterparts. This made it possible to verify each of the design steps individually due to this correlation.
Another idea evolved which was the traceability down the left side of the V. This means that the requirements have to be traced into the design of the system, thus verifying that they are implemented completely and correctly. Another feature can be observed when you compare the waterfall model to the V-model. The "Operation & Maintenance" phase was replaced in later versions of the V-model with the validation of requirements. This means that not only the correct implementation of requirements has to be checked but also if the requirements are correct. In case there is the need of an update of the requirements and subsequently the design and coding, etc. there are two options. Either this has to be treated like in the waterfall model in a never ending maintenance phase, or in going over to another V-cycle. The earlier versions of V-models used the first option. For later versions a series of subsequent V-cycles was defined, as shown in the below diagram:
This idea also correlated with the established sample phases for products as it is present in many industries. One of the cascaded V-cycles became the V-cycle of a sample phase. In addition to this the V-cycles were tailored. This means that in earlier sample phases not all the intermediate work products and process step were established to their full extend but it was simply reduced to what makes sense. By these measures the V-model became a usable process model. It does not consider every detail and possibility but evaluated over a multitude of projects in various industries it proved its usability.