A point proportion t along a curve is defined variously for different types of curves as follows:
| • | For a line segment AB, it defines the point (1t) A + tB |
| • | For a circle it defines the point on the circle which subtends angle t at the center. |
| • | For a locus or envelope, it defines the point at parameter value t. |
| 1. | Select  a point and one of the curves mentioned above. |
| 2. | Click the Point Proportional... icon  from the Constrain toolbox, or select Point Proportional... from the Constrain menu. |
| 3. | Enter the parameter or quantity (symbolic or real) in the data entry box. |
For example, in the following diagram, D is defined proportion t along AB, and E is defined proportion t along BC. The curve is the locus of F as t varies between 0 and 1.
In the following example, the curve is the locus of the point (x,x2). Tangents are created at points with parameter values x0 and x1 on this curve.
