Building information modeling (BIM) and parametric modeling are very current themes in today's AEC industry.  Instead of drawing lines, parametric building models are defined by rules and constraints, which identify aspects of the building - from physical characteristics to environmental parameters - and their relationships to each other. Creating and modifying these parameters, or the relationships between them, is an important part of the design process.  Changing a rule or constraint, or modifying a part of the model itself, almost always has implications on the entire model.  In other words, the parametric model functions a bit like a spreadsheet: changing a feature of the building is like changing an entry of the spreadsheet. In response to a change, the software regenerates the model so that pre-determined relationships are maintained, just like a spreadsheet re-calculates all of its entries.  This makes parametric building models extremely powerful design tools, facilitating the design and construction of buildings which would have been impossible only a few decades ago.