The set of pieces defined by Matt Galla's analysis of twistability that exist outside the set of real physical pieces. This is discussed in this thread but Matt who uses a totally different analysis then Andreas. He makes no assumption of a holding point and instead looks at all possible way a piece CAN move. For example he looks at pieces that move with BOTH the L and R layers on a 3x3x3. Such pieces doesn't exist in Andreas's method. But Matt goes on to prove such pieces "MUST exist mathematically". While not immediately obvious it turns out the Imaginary Pieces are a superset of the Virtual Pieces. Andreas now prefers the term NHP for Non-Holding Point over Imaginary.