The American Express pro-am format really consists of two golf tournaments going on at the same time for the 156 professionals in the field.
The 156 pros are each paired with an amateur player, forming 156 two-player pairings. Those teams are grouped into 39 foursomes over the first three days, with 13 groups playing each day on one of three courses -- host PGA West's Stadium Course and Nicklaus Tournament Course, as well La Quinta Country Club.
The American Express format
For the professionals, the tournament format is simple: the usual 72-hole, stroke-play tournament you see most weeks on the PGA Tour. Lowest score wins.
For the pro-am competition, the individual players are actually on their own. They do not form a team with their professional sherpas over the first three days. There are two competitions happening for the amateurs: a net and a gross competition. On each hole, then, an amateur has two scores: their actual score on the hole (gross) and their score minus their handicap for that hole (net). The total net score for a round is the gross score minus whatever handicap strokes are individually allocated to each player.
The American Express pro and pro-am cut rules
After the first three rounds are completed, the cut rule kicks in, dropping the field to the top 65 pros and ties and six amateurs which advance to Sunday's final round at PGA West's Stadium Course.
Prior to 2020, if more than 78 players made the 54-hole cut, then the cut was instead to the number nearest 70 for the final round. It was kind of like getting MDF'd when 78 or more players made the 36-hole cut at other PGA Tour events, except it happened after 54 holes.
For the amateurs, the six players moving on to Sunday are split evenly between the net and gross competitions. The top three players in each competition after 54 holes move on to Sunday at PGA West, and they're repaired with a new pro to play off the 10th tee for the final round.