The small bumps or ridges found on the F and J keys
on a computer keyboard are to help users correctly
position their left and right hands without having to
look at the keyboard. For example, without looking at
your hands, you can feel the correct position of your
left index finger on the F and the right index finger
on the J by feeling the small bumps on the keys.
Once you're hands are on the home row key position,
you should be able to locate the remainder of the
keys on the keyboard.
source
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