The first little gadget I find very useful, particularly when drawing street scenes of any kind, is something to help "CHECK ANGLES". There are times when perspective can be a bit of a ....let's be polite and say "challenge". Particularly if extending your lines to find eye level and vanishing point takes you right off your canvas, as so often happens. Well, even if you know none of the rules of perspective, you can get things looking right in your picture if your angles are right. So, anything which helps is worth its weight in gold.
It's a FOLDING RULER. Virtually weightlesss, very inexpensive, and extremely handy. When folded shut, it is a good 6" ruler.
So - How to use it to check angles?
It is always best to train yourself to work freehand, as it were, because it is like exercise, the more you do, the stronger your muscles become, and by working from your subject, and drawing what you see, using a bit of measuring to check proportions, the stronger will your eye become. So begin your drawing FREEHAND, just eyeballing your subject.
My folding ruler (found in the kids' section in the stationery store) is see-through plastic, very helpful. Its central pivot is strong and stiff, so it stays nicely in place. So...lets say I wanted to check angle a-b of this stone plinth. sorry, I cannot take you to the place I saw it, so you have to imagine this in real life. I began with this drawing, freehand.