A three tier architecture adds and extra level in the middle of the two entities in figure 1.1. This may or may not be another piece of hardware, in some systems it could be an extra software component. In most enterprise-wide systems it normally takes on the form of an application server. There will be middleware both sides of the added middle tier, the names of the different tiers tend to be variable but this description comes from Barry Nance [BYTE03] and is fairly generic. “The levels of a three-tier architecture are presentation, business logic and data storage. Middleware exists between each of the tiers, connecting the presentation layer to the business logic layer and the business logic layer to the data storage layer”. With this definition Figure 1.1 can be redefined to add the middle layer, shown in Figure 1.2.  ¶ 11:40 pm