The most fundamental aspect in creating a solid model is the concept of feature-based design. In typical 2-D CAD applications, a designer draws a part by adding basic geometric elements such as lines, arcs, circles and splines. Then dimensions are added. In solid modeling a 3-D design is created by starting a base feature and then adding other features, one at a time, until the accurate and complete representation of the part’s geometry is achieved. A feature is a basic building block that describes the design, like a keyway on a shaft. Each feature indicates how to add material (like a rib) or remove a portion of material (like a cut or a hole). Features adjust automatically to changes in the design thereby allowing the capture of design intent. This also saves time when design changes are made. Because features have the ability to intelligently reference other features, the changes made will navigate through design, updating the 3-D model in all affected areas. Figure shows a ribbed structure. It consists of feature like ribs and holes.

Similarly, if a flanged part shown in Fig. is to be created, the one approach is to sketch the cross section as shown in Fig. and then revolve through $360^o$.

In typical solid modeling software the designer can create a feature in two basic ways. One is to sketch a section of the shape to be added and then extrude, revolve, or sweep it to create the shape. These are called sketched features. Another type of feature is the pick-and-place feature. Here the designer simply performs an engineering operation such as placing a hole, chamfering or rounding a set of edges, or shelling out the model.

An important component of every feature is its dimensions. Dimensions are the variables that one changes in order to make the design update automatically. When a dimension is changed the solid modeling software recalculates the geometry. Design of a part always begins with a base feature. This is a basic shape, such as a block or a cylinder that approximates the shape of the part one wants to design. Then by adding familiar design features like protrusions, cuts, ribs, keyways, rounds, holes, and others the geometry of a part is created. This process represents true design. Unlike many CAD applications in which designing means drawing a picture of the part, working with the feature-based solid modeling method is more like sculpting designs from solid material.

Features/available in typical solid modeling software are: