M words are used to control many of the I/O functions of a machine. M words can start the spindle and turn on mist or flood coolant. M words also signal the end of a program or a stop withing a program. The complete list of M words available to the RS274NGC programmer is included in table 5.
| M0 | Program stop |
| M1 | Optional program stop |
| M2 | Program end |
| M3 | Turn spindle clockwise |
| M4 | Turn spindle counterclockwise |
| M5 | Stop spindle turning |
| M6 | Tool change |
| M7 | Mist coolant on |
| M8 | Flood coolant on |
| M9 | Mist and flood coolant off |
| M26 | Enable automatic b-axis clamping |
| M27 | Disable automatic b-axis clamping |
| M30 | Program end, pallet shuttle, and reset |
| M48 | Enable speed and feed overrides |
| M49 | Disable speed and feed overrides |
| M60 | Pallet shuttle and program stop |
Table 5 M Word List