What Build simple abstract frameworks to isolate structure and location from geometric detail.
When Designers sketch. Carpenters make jigs. These acts share intent; they both abstract away inessential detail, leaving a simple framework that can be easily changed. Design sketches express structure and form. Carpenters' jigs fix locations and tool paths in space. A parametric Jig mixes both of these traditions. Use this pattern when you want to quickly make and modify a simple version of your design and develop detail later.
Why Most models contain many elements and few controls. The logic of the controls is typically simple. A Jig is a simplified model that allows you to understand how the controls work without the distracting detail and slow interaction implied by a larger model. Jigs can be changed easily compared to complex models. Once developed, a Jig can be reused in other contexts, but only if it is can be isolated from the rest of the model. Jigs are like abstracting controllers, but are more specialized (they abstract a particular design), typically describe the whole design and are embedded within the design rather than being separated from it. The design is built directly on top of the Jig
How A Jig should appear and behave like a simplified version of your end goal. An physical example is the strongback and stations used in building a small boat. The stations locate and support the hull when it is being constructed. Fairing, the process of making the hull smooth and continuous, can be done much more simply with stations than with a complete hull. Jigs are like construction lines in that they help locate elements. They are unlike such lines in that they are linked to the controls that enliven the parametric model.
To make a Jig, you need to understand the parametric behaviour you want and how the Jig will be used to define the complete model. A good Jig typically has relatively few geometric inputs (for example, points, lines, planes, coordinate systems) and each of these is carefully named. The small number of geometric inputs allows you to easily locate the Jig. The names are the primary means by which you will understand the Jig when you (or someone else) reuse the Jig in the future.
Use the internal structure of the Jig to capture intended logical behaviour For example, if the depth of a truss is proportional to its span, a Jig might comprise a line and a variable whose value is proportional to the length of the line.