The set of pieces defined by Andreas Nortmann's analysis of twistability that exist outside the set of real physical pieces. This is discussed in this thread. His analysis looks at the stationary point (aka the holding point) under all possible sets of linearly independent twists for a given puzzle. In other words it looks at all the possible ways a piece can NOT move. This set of pieces is a subset of the Imaginary Pieces. Andreas himself has since moved away from using the term virtual and now prefers the term ZHP for Zero volume Holding Point.