We investigate the expressiveness of backward jumps in a framework of formalized sequential programming called program algebra. We show that-if expressiveness is measured in terms of the computability of partial Boolean functions-then backward jumps are superfluous. If we, however, want to prevent explosion of the length of programs, then backward jumps are essential.6.-7. the termination instructions !t, !f which prescribe successful termination and in doing so deliver the Boolean value t and f, respectively.Complex instruction sequences are obtained from primitive instructions using concatenation: if I and J are instruction sequences, then so is I; J which is the instruction sequence that lists J's primitive instructions right after those of I. We denote by IS(A) the set of PGLB bt (A) instruction sequences.Thread algebra is the behavioural semantics for PGA and was introduced in e.g. [1,3] under the name Polarized Process Algebra.