Precision manipulation in surgical simulation poses special requirements on a VR system. One such requirement is unification, i.e. that the user manipulates a virtual object where it appears. We have designed Cubby, a system of which the visualization part was presented at MMVR97 [3], for precision manipulation and unification. In an experiment Cubby was compared with a single screen head-tracked display, both in a unified and a non-unified version. The results show that subjects could manipulate virtual objects in Cubby with significantly higher accuracy and preferred it to the single screen head-tracked display. Though no significant difference in accuracy between the unified and non-unified conditions could be shown, subjects did prefer the unified over the non-unified condition within both setups.