Guide posts were erected at crossroads and were marked with a letter indicating the next town. An Act of Parliament in 1696 gave Justices the power to erect guide posts although track markers had been in use for many years previously. The 1765 Donn map of Devon shows several tracks crossing the centre of Dartmoor along with the location of the guide posts. Many guide posts have been removed and used as gateposts but the remaining stones match the sites indicated on Donn's map.
The photograph shows one of the guide stones marking the old track between Tavistock and Ashburton. These stones are marked with an incised letter "T" on the side towards Tavistock and the letter "A" on the side towards Ashburton. For this reason, they are also known as T-A stones.