The use of computers for processing three-dimensional (3D) engineering drawings has been investigated in this paper. The essential properties of a hierarchical data structure are pointed out and employed to provide a natural computerized data structure for representing the geometry of the drawn part The intrinsic flexibility of that structure makes it possible to keep track of a full 3D description of the part under consideration. Although a hierarchical data structure is not the neatest structure for expressing geometrical properties, it may form a convenient framework for data processing. It also has the advantage of being embedded in high-level languages. In the system presented here auxiliary light buttons are introduced to facilitate unambiguous stepping up or down through the structure hierarchy. Consequently the system can adequately transfer program control to a particular part of the graphic system to perform the exact specific tasks. The given system handles solid objects with plane surfaces and can be extended for processing cylindrical surfaces when they ere represented by their canonical parameters. Such a system will be economical and quite convenient for drawings that are widely used and subjected to frequent minor changes.