The principal products we have included in this latest edition of the Guide are summarised in Table 1‑1. A number of these products are free whilst others are available (at least in some form) for a small fee for all or selected groups of users (e.g. GWR). Others are licensed at varying per user prices, from a few hundred to over a thousand US dollars per user. Our tests and examples have all been carried out using desktop/Windows versions of these software products. Different versions that support Unix-based operating systems and more sophisticated back-end database engines have not been utilised. In the context of this Guide we do not believe these selections affect our discussions in any substantial manner, although such issues may have performance and systems architecture implications that are extremely important for many users.