Abstract: One embodiment is an apparatus for capturing marker stroke locations, including erasures. The marker includes a transmitter of acoustic pulses and an erasure cap detector that detects when a removable erasure cap is attached to the marker. A receiver includes two or more sensors to sense the acoustic pulses, the receiver operable to determine and track marker locations. When the removable erasure cap is attached to the marker, the marker locations are erasures, and when there is no erasure cap attached, the marker locations being tracked capture strokes. Another embodiment is a method of capturing marker stroke locations, including erasures, using the marker, the removable erasure cap for erasures, and the receiver.