This paper explains concurrency issues present when modeling software systems using ordinary place transition nets. Normally it is assumed that ordinary (elementary) Petri net structures are concurrent or concurrency free. However it can be shown that concurrency does not depend only on the structure of a Petri net but on the distribution of resources in the net. A basic algebraic notation is used for representing concurrency. Examples are given showing different types of concurrency that can be detected in ordinary Petri net structures. These are classified into i) dependent and ii) temporal (weak) concurrency.