Some of the oldest POW embroideries date from the Napoleonic wars and were made by soldiers to supplement their food rations. A later example of an English POW embroidery was made by Major Alexis Casdagli during the Second World War (1939-1945). The embroidery contained hidden messages worked in Morse code.
See also the TRC Needles entry on sailor's woolwork.
Digital source of illustration (retrieved 29th May 2016).
GVE