If you want to mark lines parallel to an edge, you need a panel gauge.
They're easy to make out of any scraps you have lying around the workshop, and I'll explain how in the following steps.
If you would like to see this one being made, then I have a video here:
A SketchUp model is also available to download from my website:
Step 1: Make the Stock
The stock is the main body of the gauge. It has a rebate (rabbit) that acts as a fence against the edge of the panel being marked out, and a through mortise that holds the beam. A small wedge holds the beam in place, once the desired gauging distance has been set.
Take a 10" to 12" length of 3"x2" hardwood, and plane two adjoining surfaces at ninety degrees to each other. One will be the base of the stock, the other will be the face.
Mark in a 1/4" x 1/4" rebate where the base meets the face. This will form the fence. By using a rebate, the gauge will sit on the edge of a panel without the risk of pivoting on the beam.
Mark in a 1/2" x 1-1/2" mortise, with an additional 3/8" extension on one end for a wedge, in the centre of the face and abutting the rebate.
To lighten the stock, mark in a profile that is thick around the mortise, and narrow towards each end. You can copy mine, or make something up yourself.
Knife in the mortise and extension, and then chop the mortise right through. When you're done, extend the mortise on the face out to the 3/8" extension, and on the opposite face by 1/4". This produces a sloped recess into which the wedge can be slid to tighten up the beam.
Cut the rebate, which should just meet the edge of the mortise.
Finally cut away all the waste to reveal the finished stock.
Step 2: Make the Beam
Plane a 2' to 3' length of hardwood to 1/2" x 1-1/2".
Fit to the mortise thickness, before running a small bevel on all edges.
Drill a hole through the beam to take a pencil, about 2" from the end, and make a saw cut through this hole from the end, and extending to about 4". Check that by squeezing the end together, a pencil is firmly gripped in the hole. If not, widen the saw kerf using a thicker saw.
Finally drill a cross hole about 1" from the end to accept a bolt with a knurled nut.
Step 3: Final Fittings
Saw out a headed wedge, and size to fit the mortise.
Thickness the wedge so that it tightens up on the beam before the head reaches the mortise, but after it's tip emerges from the other end of the mortise by at least 1/4". This will allow for both setting and releasing with a light tap.
Install a bolt with a knurled nut into the tightening hole in the beam, and grip a pencil ready to get to work.
Thanks for viewing my Instructable, I hope you enjoyed it.