Flat Holm is a Welsh island lying in the Bristol Channel approximately 6 km (4 mi) from Lavernock Point in the Vale of Glamorgan. It includes the most southerly point of Wales.

Telegraphy is the long-distance transmission of messages where the sender uses symbolic codes, known to the recipient, rather than a physical exchange of an object bearing the message. Thus flag semaphore is a method of telegraphy, whereas pigeon post is not. Ancient signalling systems, although sometimes quite extensive and sophisticated as in China, were generally not capable of transmitting arbitrary text messages. Possible messages were fixed and predetermined, so such systems are thus not true telegraphs.