Final answer:
The attachment that allows you to cut all the hair to one exact length when using electric clippers is called a guard.
Step-by-step explanation:
In the context of using electric clippers, the attachment that allows you to cut all the hair to one exact length is called a guard. The guard is a detachable plastic comb-like accessory that is attached to the clippers' blade to determine the length of the hair being cut. It has different numbered sizes or lengths, such as 1, 2, 3, etc., indicating the desired length of the hair left after cutting.