In 2006, the US Open became the first Grand Slam tournament to implement instant replay review of calls, using Hawk-Eye. Available only on the stadium courts (Ashe and Armstrong), each player is allowed two challenges per set plus one additional challenge during a tiebreak, but is not penalized with the loss of a challenge if the challenge turns out to be correct. Once the challenge is made, the official review (a 3D computer simulation based on multiple high-speed video cameras) is shown to the players, umpires, and audience on the stadium video boards and to the television audience at the same time. The system is said to be 100% accurate to within 5 millimeters. During the 2006 US Open, 30.5% of men's challenges were overturned and 35.85% of women's challenges were overturned.