Stabilizers or stabs for short. They support the larger keys on the keyboard. They come in a variety of sizes and mounting styles. There are different brands of stabilizers but they can be classified into these categories:
Image via Ali Express
Plate mounted stabs mount on the plate. They are considered the inferior stab because when not modded they tend to rattle when you type on them.
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Screw in stabs mount on the PCB. This mount style is the best because it is the most secure way to mount the stabs and does not rattle when you type on the keyboard.
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Snap in stabs mount on the PCB. They are less secure then screw in stabs. They tend to pop out when you remove key caps or when handled roughly.
Stabilizer are measured with based on the sizes of keycaps. 1u is the width of a single keycap. The 3 most common stabilizers are 7u, 6.25u and 2u.
7u is used for non-standard keyboard layouts. A standard spacebar is 6.25u. 2u is used for all other larger keys like the shift key or backspace key.
A full sized keyboard will use:
6.25u x 1