The Four-Function Ruler pen from Uncommon Goods was designed by Adrian Olabuenaga to be the world’s thinnest four-function pen. There is a pen with black ink, a highlighter, a mechanical pencil, a PDA stylus, and a hidden eraser under the cap. The pen is also marked with inches and centimeters. The Four-Function Ruler pen is $35.00. Refills are available from Uncommon Goods for $3.00 to $9.00.