Why do the F and J keys have bumps?
The short answer
The bumps on the F and J keys are tactile landmarks. They help you find the basic hand position without looking down. Place your left index finger on F and your right index finger on J, and the other fingers can line up with the home position.
How do they help?
While typing, your fingers and hands move slightly. Feeling for a bump lets you check your position and return to the F or J area without looking at the keyboard.
On an English letter layout, the left-hand fingers rest around A, S, D, and F, while the right-hand fingers rest around J, K, L, and semicolon. This home position gives your fingers a reference for moving to other keys and returning.
Does every keyboard have them?
Many physical keyboards have a small bar or dot on F and J, but they do not all use the same shape. The position and form can vary with the layout and design. A touchscreen has no physical bump.
Does the number 5 key have one too?
On a numeric keypad, the central 5 key often has a bump or depression. It provides a similar tactile landmark for finding the numbers. This feature also varies by keyboard.
Key point
The bumps are not parts that automatically make typing faster. They are reference points that help you orient your hands by touch and reduce the need to look down. They can be useful even if you do not type with all ten fingers.