Solid-modeling software can be classified according to different quadrants. The “X-axis” represents whether a geometry model is generated from a set of instructions (history-based), or depicted with an explicit, “dumb” geometry (geometry-based). The other axis represents how designers interact with the model, either by driving it primarily using hands-on tools (tool-driven), or by editing constraints (constraint-driven). Software such as SolidWorks Instant3D provides tools that let users interact with a 3D model as part of a traditional history-based modeler. Other programs such as NX and Solid Edge provide both a history-based and a constraint-driven direct modeler. The pure direct modelers in the upper-right offer an unadulterated direct modeling experience. (ST stands for synchronous technology.)