For most skiers, over-the-calf socks are plenty tall and are the go-to choice. But knee-high ski socks—reaching just below the knee at 16 to 20 inches from the heel—do have a place for certain situations and riders.
Knee-high socks are popular with ski racers who want full compression coverage along the entire lower leg, which can help with blood flow and reduce fatigue during high-intensity runs. They're also a great option if you run cold or ski in extreme conditions, since the extra coverage traps more warmth. Some backcountry skiers prefer them for long tours in bitter temperatures.
The trade-off is that the taller cuff can feel restrictive or bunch up behind the knee, creating pressure points. If you go with knee-highs, make sure they fit snugly and don't sag or wrinkle. For typical resort skiing, over-the-calf is usually the more comfortable and practical choice.