Details of an improved Jacob staff are put into the literature as a reference for all field geologists. The new Jacob staff is built around a standard Abney-level head with an auxiliary clinometer and spirit level and has the following chief advantages over other Jacob staffs: 1) it is useful in traversing along as well as across strike, and 2) the length of the staff is adjustable. The various components are detachable. The procedure for measuring sections across strike either up or down dip is quite simple. It is hoped geologists will find this new field aid of use in field mapping.