This paper is intended to give a brief description of the strands interface and how it can be used. The interface separates scheduling from implementation of threads, and subjugates both to user control. It should be clear that all the failure modes are ``safe,'' that is, any failure by any system handler to carry out the necessary duties only results in a damage to the strands in question. This is an integral part of the fault isolation properties that an extensible system such as SPIN needs to insure.