When we use a reference to endpoint objects, or center, for example, what we are actually doing is forcing the new object to share a point of its geometry with another object already drawn. If we use a “Parallel” or “Perpendicular” reference, the same thing happens, we are forcing the geometric arrangement of the new object with respect to another, so if it is not parallel or perpendicular, depending on the case and among other options, that new object cannot be created
"Parametric Restrictions" can be seen as an extension of the same idea that inspires references to objects. The difference is that the established geometric arrangement remains a requirement that the new object must meet permanently, or rather, as a restriction.
Thus, if we establish one line as perpendicular to another, then no matter how much we modify that other line, the object with restriction must remain perpendicular.
As is logical, the application of a restriction makes sense when we modify an object. That is, without restrictions we can make any changes to a drawing, but as these exist, the possible changes are limited. If we are going to draw with Autocad an existing object that does not require any change, then it does not make any sense to apply some parametric constraint in that drawing. If, on the other hand, we are making a drawing of a building or a mechanical part whose final form we are still looking for, then the parametric restrictions are of great help, since they allow us to fix those relations between the objects, or their dimensions, that our design must comply.
Put another way: parametric constraints are a great tool for design tasks, because it allows us to fix those elements whose dimensions or geometric relations must remain constant.
There are two types of parametric constraints: Geometry and Dimension constraints. The former specify the geometric constraints of objects (perpendicular, parallel, vertical, etc.), while dimensional constraints (distances, angles and radius with a specific value). For example, a line should always be 100 units or two lines should always form an angle of 47 ° degrees. In addition, dimension constraints can be expressed as equations, so that the final dimension of an object is a function of the values (variables or constants) of which the equation is composed.
Since we are going to study the objects editing tools from the 16 chapter, here we will see how to create, view and manage the parametric constraints, but we will return to them in that chapter.