The Mars® 571 is a set of three French curves — shapes A, B and C — in transparent blue plastic. A French curve is what you reach for when a curve has to be smooth and repeatable rather than freehand: you match a segment of the template to the run you want and draw against it. Each shape carries an inking edge, so a technical pen rides clear of the paper and the line does not bleed underneath. Supplied in a wallet.

• Wallet containing 3 French curve shapes: A, B and C
• Transparent blue plastic — the drawing stays visible underneath
• Inking edge on each shape, lifting a technical pen clear of the paper
• For smooth, repeatable curves that a freehand line cannot hold