A situation that may arise when two (or more) separately active processes compete for resources. Suppose that process P requires resources X and Y and requests their use in that order, and that at the same time process Q requires resources Y and X and asks for them in that order. If process P has acquired resource X and simultaneously process Q has acquired resource Y, then neither process can proceed, each process requiring a resource that has been allocated to the other process. On larger systems containing more than two processes and more than two resources, it is still possible for deadlock to develop although its detection may be more difficult.