  Threads as considered in basic thread algebra are primarily looked upon as
behaviours exhibited by sequential programs on execution. It is a fact of life
that sequential programs are often fragmented. Consequently, fragmented program
behaviours are frequently found. In this paper, we consider this phenomenon. We
extend basic thread algebra with the barest mechanism for sequencing of threads
that are taken for fragments. This mechanism, called poly-threading, supports
both autonomous and non-autonomous thread selection in sequencing. We relate
the resulting theory to the algebraic theory of processes known as ACP and use
it to describe analytic execution architectures suited for fragmented programs.
We also consider the case where the steps of fragmented program behaviours are
interleaved in the ways of non-distributed and distributed multi-threading.
