"Bill" writes:

Beginner's question: Say I use a fine square to mark a 3 or 4 inch
line "perfectly", with an awl or knife. How can I extend that line to, say,
10 or 20 inches with accuracy that would please Starrett's customers?

Well, a Starrett fan would have a 24" combination ruler. But I would
either make additional marks along the edge, and especially the other
end of the piece of wood, extending the line. and then use a
straightedge or framing square.