The command block controls the machine tool through the use of letter address commands.  Some are used more than once, and their meaning changes based on which G-code appears in the same block.

Codes are either modal, which means they remain in effect until cancelled or changed, or non-modal, which means they are effective only in the current block.  As you can see, many of the letter addresses are chosen in a logical manner (T for tool, S for spindle, F for feed rate, etc.).

The table below lists the most common Letter Address Commands.  In the CNC class, the programs will not use all of the letters.  Other letters will be used in the Advanced CNC class later.