Line judges stand at the corners of the court, typically one per corner at higher levels of competition, and are responsible for calling whether a ball that lands near a boundary line is in or out of the court, along with related calls such as whether a ball crossed the net outside the antenna or whether a server's foot touched the end line during a serve. Because line judges are positioned specifically to watch the boundary lines from a close, low angle, they are typically better placed to make these precise calls than the first or second referee, who must track the entire court and cannot focus exclusively on any single line.
Line judges signal their rulings using flags rather than hand gestures alone: a flag pointed down signals that a ball was in, a flag raised vertically signals out, and touching the top of a raised flag with the free hand indicates that a player touched a ball on its way out of bounds, which changes which team is awarded the point.
A check the first referee can still overrule
Although line judges make the initial call on their assigned boundary, the first referee retains ultimate authority and can, in rare cases, overrule a line judge's signal if it is clearly and obviously mistaken, keeping final responsibility for every ruling with a single official even within a multi-person officiating crew.
Fewer line judges at lower levels of the sport
Top-level FIVB and Olympic matches typically use four line judges, one for each corner, but many lower levels of organized volleyball — school, club and recreational competition — use only two line judges, or occasionally none at all, relying instead on the referees or even player self-officiating for boundary calls. This scaling of the officiating crew's size to match the level of competition is a practical necessity given the cost and availability of trained officials, but it does mean boundary-call accuracy realistically varies quite a bit across different levels of the sport.
