Concurrent computations have been modeled using partial orders in both event based and state based domains. We give necessary and sufficient conditions on partial orders for them to be valid state based or event based models of concurrent computations. In particular, we define notions of width-extensibility and interleaving-consistency of partial orders, and show that a partial order can be valid state based model of a concurrent computation iff it is width-extensible. Distributed computations that involve asynchronous message passing are a subset of concurrent computations. For asynchronous distributed computations, a partial order can be a valid state based model iff it is width-extensible and interleaving-consistent. We show a duality between the event based and state based models of concurrent computations, and give algorithms to convert partial orders from the event based domain to state based domain and vice-versa. / text