Guidewords are the tools that are used to systematically direct the HAZOP Study. They are words or phrases that, when considered together with a parameter, form a hypothetical deviation for the Team to consider. The basic guide words are: NO, MORE, LESS, AS WELL AS, PART OF, REVERSE and OTHER THAN.

Guidewords are combined with process Parameters such as: FLOW, PRESSURE, TEMPERATURE, and LEVEL, or General parameters, such as MAINTENANCE, START-UP / SHUTDOWN and others, describe operations more so than physical aspects of the process.

The combination of Guideword and Parameter gives a Deviation Matrix which identifies departures from the design intention. This is the starting point for the HAZOP-Team to brainstorm causes, consequences, safeguards and residual risk of the identified hazards.