The thangail was a defensive formation used by the soldiers of the Númenórean Realms. It was formed from two closely-pressed ranks presenting a barrier of shields, solidly enough to withstand the onslaught of their enemies. The thangail was designed to be flexible, and could bend at one or both of its ends to adapt to enemy tactics, even curling round to meet itself and so form an unbroken circle of shields.
The only record of the thangail in actual use was by Isildur, who ordered one drawn up when attacked by Orcs in the incident known to history as the Disaster of the Gladden Fields. However, there is reason to think that it originated with the Elves, especially as the two main Elvish languages had quite separate names for the formation. Thangail is a Sindarin word, whereas in Quenya the same tactic was referred to as a sandastan.