Guide Draw

Guide Tool

Use this tool (Tools menu) to interactively create guidelines and subsequently adapt them (using the Handles to snap to other vertices, etc.). See also above.

Guidelines can be create as follows (of course they work in combination with all Snap options):

If you press the Ctrl while editing guidelines using the Guide tool, the guidelines will be duplicated. This lets you very quickly create such constructs as below (simply Ctrl+drag a Handle):