From FreeCAD Documentation
- Make sure there is no selection.
- There are several ways to invoke the tool:
- Press the Constrain block button.
- Select the Sketch → Sketcher constraints → Constrain block option from the menu.
- introduced in version 1.0: Right-click in the 3D view and select the Constrain → Constrain block option from the context menu.
- Use the keyboard shortcut: K then B.
- The cursor changes to a cross with the tool icon.
- Select a single edge.
- A constraint is added.
- Optionally keep creating constraints.
- To finish, right-click or press Esc, or start another geometry or constraint creation tool.