Optical tracking with fiducial markers is commonly used in Augmented Reality (AR) systems. AR systems that rely on the ARToolKit [Marker Tracking and HMD Calibration for a Video-based Augmented Reality Conferencing System] are prominent examples. The information obtained by the tracking subsystem are widely used in AR, e.g. in order to calculate how virtual objects should be located and oriented. The results of extensive accuracy experiments with single markers are reported and made operational by the definition of an accuracy function. The results show a specific distribution of tracking accuracy dependent on distance as well as angle between camera and marker. This insight is applicable for designing the set-up of AR applications in general that rely on optical tracking.