This paper introduces a prediction tool - Sirolan-Yarnspec - which can be used in electing tops which will meet the spinner's needs. Yarnspec allows the user to gauge expected spinning performance and yarn properties for any given pure wool worsted yarn. The improved understanding of how similar spinning performance can be achieved with different top specifications should give the topmaker greater flexibility in meeting his customer's needs at an optimum price. The top fibre properties which affect spinning performance are shown to be, in order of importance, diameter (D), length (H), strength and diameter distribution (CV D). The importance of length distribution (CV H) seems to be over-rated and whilst lower crimp gives better yarn and spinning performance, it will not usually be a consideration. The potential importance of fibre strength has led to the development of an improved instrument - Sirolan-Tensor. Currently, both Yarnspec and Tensor are undergoing mill validation.