The two screws labeled "L" and "H" are used to adjust your limits. Basically, they determine how far in either direction the chain can travel. Unless you're having issues with the chain falling out, there is no reason to adjust these screws on the rear derailleur (as they were likely set properly at the factory). However, if your bike was in an accident or you installed a new derailleur, these screws may need to be adjusted.









If you keep slipping a chain, check your limit screws.
If you can't shift into the highest or lowest gear, check limit screws.
If your chain is grinding against your frame, check limit screws., Of course, you would turn it counter-clockwise if the chain is getting stopped too early. The high limit is for the smallest gear in the rear derailleur., Again, turn it the opposite way (counter-clockwise) if the limit is pushed too far in. On the rear derailleur, the low limit is for the biggest gear., Once you've got the limits where you want them, make sure they are lined up nicely. You should see the jockey wheel (metal arm with small cogs under the derailleur) lined up right underneath the chain.
 If you are in the furthest gear, you will see the derailleur moving as you adjust the screws. So, if you need to adjust the low limit, shift to the gear on the far left, the biggest one. Adjust both screws a 1
Only adjust your limit screws if the chain is popping out on either side of the derailleur.
Turn the "H" (high) limit screw clockwise to prevent the chain from moving too far to the right, away from the bike.
Turn the "L" (low) limit screw clockwise to prevent the chain from moving too far to the left (and potentially into your wheel).
Shift to the high and low gears, visually confirming that the derailleur is lined up with the center of the gears.
Test both screws and watch to see when the derailleur moves if the H and L are no longer marked.
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