This is a fully parametric parallel line drawing tool you can use in the classroom or at home to quickly and easily draw things that require consistently-spaced parallel lines, including musical staffs, tablature (TAB), guitar chord diagrams, etc, as well as handy things for math like Cartesian coordinate graphs and multiplication tables.
If you know a music or math teacher, or even a mathemusician, ask them how wide their writing implements are and how far apart they like their lines, and you've got a great gift idea.
The OpenSCAD file is fully parametric, well-commented, and easy to modify. You can change the number of lines, the distance apart, pen diameter, pen grip length and thickness, amount the grip "wraps", extension from handle, extension thickness, handle height/thickness/length, and even handle curve radius.
With a little tweaking of the parameters, you can set it up for chalk/blackboards, street chalk, pencils, different dry erase markers, crayons, or just about any other cylindrical writing implement.
I plan to add a keyhole-style mounting system because it also seems to serve quite well as a pen holder -- and if you hold the pens at the cap, you'll never forget to recap them because they won't stay held if you don't.
The default settings (and the STL) are set for the ubiquitous Expo dry erase markers, spaced 30mm apart. Modify these parameters and re-render for other things like chalk, crayons, pencils, etc.
2) Snap the writing implements into place.
3) Hold the jig between your thumb and index finger, and pull it across the surface to draw the lines.
The example pictured (printed from the STL) used only about 1.4m of 3mm filament (about US$0.25) and holds up nicely printed with two perimeters, two solid layers, 10% infill, and 0.4mm layer height.