Slope is a course’s difficulty relative to the course rating. The course rating tells how an exceptional, or scratch golfer, should be able to play, and is expressed in numbers that look suspiciously like a score, as in 71.9. That’s what a scratch golfer should score on that particular course.
The slope says how much harder that course would be for the average golfer.
Slope ranges from 55 to 155, with an average being 113. Thus, a course with a slope of 120 is more difficult than average, while one with a 100 is easier than average.
Slope is used to calculate a player’s handicap because it compensates for the varying difficulties of courses. A player who shoots a 90 on a 155 slope course is doing better than one who shoots 80 on a 55 slope course.
The term slope apparently refers to the slope of a line if a player’s scores were graphed with scores on one axis and course difficulty on the other.
The USGA uses trained Course Rating Teams to calculate a course’s slope. It is based on a careful measurement of distances and is adjusted for roll, elevation, hole designs, prevailing wind, altitude and obstacles that may affect play. They even factor in psychology.