Reference designators are used in schematics and printed circuit board layouts to label components. Reference designators are commonly alphanumeric; they consist of a letter indicating the general category of component followed by a number. Some commonly used reference designator letters are:

Reference designators are essential when moving from a schematic to a board layout. You might not have fixed in your mind what the package size and shape of a given component is, so in order to recognize it and place it appropriately, you will need to have some means of identifying the component.

Many software packages (such as Mentor Graphics) give the user the option of either assigning reference designators manually, or letting the software assign them automatically. The automatic assignment feature is useful for components such as resistors, where there is likely to be a large quantity of identical parts.