The simplest is the T-Square. Remember that straight edge I mentioned about the drawing board? Well, here's where it comes in. You take the "T" of the T-Square and hold it against the straight edge of the board. You can slide the T-Square up and down the edge and draw as many straight lines you wish. Here's the best part; every one of these lines will be parallel!
The next option is a parallel ruler attached to the drawing board. The picture to the right is of a portable board with the ruler. The ruler is held in place with small cables and pulleys. These allow the ruler to move up and down. Very cool. These come in various sizes. You can also get the ruler in a kit and attach it to any drawing board.
If only all our lines would run in one direction. But alas, we need to draw a lot of rectangles. Platforms, flats, doors, windows, lots of rectangles. All of our vertical lines would want to be perpendicular to our horizontal lines. That is, a 90 Degree angle. But you knew that, I know. Just making sure.
A triangle is the best choice. These two triangles both have a 90 degree angle. But they are different. The one on the left is a 30-60-90 degree and the other is a 45-90 degree. Having both can give you all the degrees of the compass, but at 15 degree increments.