In theory any configuration or reinforced cast in place concrete can be formed with site built formwork. But in practice custom made formwork can often provide advantages not found in built up or standard prefab forms. But first, before deciding which type of formwork- job built, prefab or custom designed- is best for a particular job, the contractor should prepare a written analysis. A working knowledge of pressure, forces, and loads will be most helpful in preparing this analysis. The basic concern should encompass two main areas: adequate strength and economy. If after the analysis has been made standard prefabricated forms cannot be used or be modified to fit the job, and if site built forms appear to be out of the question, special or custom made forms may have to be used. In developing custom forms, the contractor may consider two avenues: using his own staff to design the forms or working with a form manufacturer's engineering department. The latter has advantages to it, since the manufacturer of prefabricated forms accumulates a great deal of experience over a period of time. Using the form manufacturer's service, the contractor will be able to gather figures on methods and equipment before actually obligating himself to any cost. Once the job has started, it is advisable to keep cost and time studies on each operation that is involved. This information would be very helpful to the contractor on future jobs requiring special formwork and even of a job where the forms will have repetitive re-use.