Slalom is regarded as the most technical of the alpine ski disciplines.
A course is constructed by laying out a series of gates. Gates are generally formed from either two red poles or two blue poles. The skier must pass between the two poles forming the gate. (Strictly, the tips of both skis and the skier's feet must pass between the poles.)
For slalom the vertical offset between gates is around 9m and the horizontal offset around 2m, although these figures have changed significantly in recent times because of the huge technical developments in ski equipment which have revolutionised the sport. The gates are arranged in a variety of differents configurations to challenge the competitor. The world wide govening body, FIS (Federation Internationale de Ski) has a set of regulations detailing what configurations are allowed / mandated for an official course
Because the offsets are relatively small in slalom, skiers take a fairly direct line and often knock the poles out of the way as they pass. In modern slalom, a variety of protective equipment is used such as shin pads, hand guards, helmets, face guards.
The rules for the modern slalom were developed by Sir Arnold Lunn in 1922 for the British National Ski Championships, tried by the FIS in 1928, and adopted for the 1936 Winter Olympics. Under his rules, the gates were marked by pairs of flags rather than single ones, were arranged so that the racers had to use a variety of turn length to negotiate them, and scoring was on the basis of time alone, not time and style.