From FreeCAD Documentation
- Make sure there is no selection.
- There are several ways to invoke the tool:
- Press the Constrain tangent or collinear button.
- Select the Sketch → Sketcher constraints → Constrain tangent or collinear option from the menu.
- introduced in version 1.0: Right-click in the 3D view and select the Constrain → Constrain tangent or collinear option from the context menu.
- Use the keyboard shortcut: T.
- The cursor changes to a cross with the tool icon.
- Do one of the following:
- Select two edges. The edges can be any edge except a B-spline.
- Select a point and two edges (in that order).
- Select an edge, a point and another edge (idem).
- A Tangent constraint is added. If a point and two edges have been selected, up to two Point on object constraints can also be added. See Examples.
- Optionally keep creating constraints.
- To finish, right-click or press Esc, or start another geometry or constraint creation tool.