This study isolates the most prevalent devices in trombonist Hal Crooks' style of improvising over a two year recording period, and discusses the use of these devices in relation to a variety of familiar jazz analysis categories. In order to gain an insight into the improvisational style of Crook it is necessary to examine the recurring musical ideas common to his improvisations. It is t the intention of this study to try to determine Crook's intuitive processes when improvising, but merely to categorise his most abundant and therefore, we assume, most practiced and consciously chosen structures. The method for analysis used in this paper will take the form of a survey of core material, ie, categorising similar fragments from each of the solos into isolated cells to be compared. Each cell will then fit into a larger category such as Harmonic, Melodic or Rhythmic devices. With similar devices highlighted side by side it should become apparent as to Crook's more favoured approaches to musical situations. This data is then compared to devices discussed in Crook's own improvising literature. The analysis is supported with three transcriptions which provide the data for this study.