It's called the 3-2-1 rule, and it's the easiest way to remember the regulation. To recap, if the weather at your destination isn't at least 3 SM of visibility and 2000' AGL ceilings from 1 hour before to 1 hour after your ETA, you need to file an alternate.

The 1-2-3 Rule is a fundamental FAA regulation (specifically FAR 91.169) that pilots must use when filing an IFR (Instrument Flight Rules) flight plan to determine if they need to designate an alternate airport. The rule states that an alternate is required unless the weather at the destination is forecast to have: (1) a ceiling of at least 2,000 feet above the airport elevation, and (2) visibility of at least 3 statute miles, for a time window of at least 1 hour before to 1 hour after the estimated time of arrival (ETA). If the forecast shows that either the ceiling or the visibility will fall below these "2,000 and 3" thresholds at any point during that three-hour window, the pilot must legally list a second airport where they can land if the weather at the primary destination becomes too poor to attempt an approach. This rule is a critical safety margin designed to ensure that every instrument flight has a "Plan B" already vetted and fueled for in case of unexpected weather deterioration.