Woo designed a pen attachment that can help you draw straight lines. Zalled the"Constrained / okay guy :: invention :: line :: pen :: funny pictures / funny pictures & best jokes:
Constrained Ball
Design of a new object/product - Modified "Constrained Ball" | Freelancer
This small metal trinket lets you draw straight lines without a ruler - Yanko Design
SEMI-CONSTRAINED BALL AND SOCKET JOINTS - diagram, schematic, and image 46
Constrained Ball - Concept Line Maker and Measurer - Make: