A beacon was a fire deliberately lit to give a warning, of smoke by day and flame by night, of the approach of hostile forces. They were always sited in prominent positions and usually as part of a group, chain, or line. They were initially bonfires of wood, furze, or other inflammable material, but later used barrels of pitch, or one or more fire baskets on a pole, or, less frequently, in a stone structure of some kind.
Although warning fires were known from Saxon times, the use of the term "beacon" as a general reference only appears in 1326; the system was still in use in 1625, but thereafter decayed although many were renovated later. Many of the sites in the south were used later, during the Napoleonic wars, but as semaphore signal stations.
The sites themselves contain little by way of dating evidence, but many beacons can be dated precisely from documentary evidence. Failing these, maps and placenames may give evidence of the existence of beacons, those that are corruptions of "watch and ward" being early examples, and those using "beacon" or "firebeacon" being later. There are many of these, usually being "Beacon" preceded by a placename.
Some were built near possible landing places, from where warning could be given to a nearby hill-top beacon and thence onward into the beacon system. Most were on prominent hill-tops, from where adjacent beacons could be seen, at distances varying between say 4km to 25km. They were always sited as part of a group or chain. There are several beacon sites known in Buckinghamshire. Ivinghoe Beacon and Beacon Hill in Ellesborough are two prominent hills with place-name and other evidence for beacons.