Conventional semantics for shared-variable concurrency suffers from the ``grain of time'' problem, i.e., the necessity of specifying a default level of atomicity. We propose a semantics that avoids any such choice by regarding all interference that is not controlled by explicit critical regions as catastrophic. It is based on three principles:

• - Operations have duration and can overlap one another during execution.
• - If two overlapping operations touch the same location, the meaning of the program execution is ``wrong''.
• - If, from a given starting state, execution of a program can give ``wrong'', then no other possibilities need be considered.