In slalom alpine skiing how do skiers know which poles to go around?

Traditionally, each gate is marked by two poles of the same color, and the racers have to ski in between two poles. Both tips and feet have to break the imaginary plane connecting between two poles. So generally, racers just go around the inside pole of each gate. To simplify the setup of the course, the outside poles are sometimes left out. The exceptions are the combinations: under gate, hair pin,flush. Unlike most common gates that are across the fall line(horizontal), combinations are made up by two poles of the same color along the fall line( vertical). Combinations have to be set up by two poles. To sum this up, go in between two poles of the same color, ski to the outside of the single poles. Watch some races and you'll learn.