How do programs like autocad?

Normally, a three-dimensional object is entered into the computer as a three-dimensional object. However, sometimes the object is such that the third dimension can be easily generated from the other two dimensions. The extrude function is one such case. This is the conceptual equivalent of squeezing material through a die of the right shape, such as the cylinder of toothpaste you get when you squeeze it through a circular hole. Pasta and seamless pipe can be produced the same way. The logical equivalent of an extruded object can be generated simply by specifying the two-dimensional shape and the length of the extrusion, then letting the computer do the rest.