You can construct a locus of points or envelope from a selected point or line, by defining a range for some constraint in the drawing. Just follow these easy steps:
| 1. | Select  the point on the drawing that will form the locus (point B in the example below); select a line, line segment, or vector to form an envelope. |
| 2. | When you click the Locus icon  , the Edit Locus dialog pops up. |
Simply fill in the values for the appropriate variable. Click the arrow key to the right of the Parametric Variable window to select from a list of all variables in the drawing. (If you entered the constraint in real terms, change it to a variable by doubleclicking the constraint .)
Click the Ok button and the locus will appear.
! Note: The locus only works if the figure’s position is fixed i.e. a point in a triangle will not work as a locus unless the location of the other two points are fixed with coordinate constraints.
The following example shows an envelope of the line DE. We use the Point proportional along curve constraint and the parameter t to position the points D and E (D is (1-t) along line AC and E is t along line AB). In the Edit Locus dialog, we create the envelope from parameter t as it ranges from 0 to 1.
You can replay the creation of the envelope with the Animation tools. Select variable t and click the Play button.