If you bicycle much using the most common shifting systems, you’ll need these. Thus far I am finding the instructions on Liv’s Cycling to be best for the rear, and wikihow or madegood.org for the front. The most important thing yet learned: instructions often get the “H” and the “L” limit screws reversed for whichever bicycle you’re on. Got to check it for each derailleur, by seeing the slight movement per half-turn of each screw. General thoughts:

1. Mounting position has to be right. This is not very confusing in the various web resources studied.
2. Each derailleur, front and rear, has an H screw and an L screw. On each, one screw sets the max deflection towards the frame, the other sets the max deflection away from the frame. These are called limit screws.
3. Cable tension can simulate one of the limit screws. Use the tension caps on the gear control, or (when they are there) the caps on the derailleurs themselves, to relieve tension, to verify that you are seeing reality of limit screws, not the simulation of the cable tension :-) Which screw tension simulates, also varies by make and model and type of derailleur.