The Gaussian sampling strategy decides to keep a point based in part on testing a pair of samples. This idea can be carried one step further to obtain a bridge test, which uses three samples along a line segment. If the samples are arranged as shown in Figure 5.29b, then the middle sample becomes a roadmap vertex. This is based on the intuition that narrow corridors are thin in at least one direction. The bridge test indicates that a point lies in a thin corridor, which is often an important place to locate a vertex.