A digital pen and paper system comprises a pen arranged to make pen strokes on a document having a plurality of regions, to detect markings on the document, and to record them in a series of samples. The system is arranged to determine from the samples the position of the pen strokes. The system has defined therein a plurality of different warning conditions that determine when a warning will be produced in response to lost samples, and is arranged to select one of the warning conditions for one of the pen strokes depending on which of the regions the pen stroke is made.