Lance is a catchall term for a variety of different pole weapons based on the spear. The name is derived from lancea, Roman auxiliaries' javelin, although according to the OED, the word may be of Iberian origin. Also compare longche, a Greek term for lance.
A lance in the original sense is a light throwing spear, or javelin. The English verb to launch "fling, hurl, throw" is derived from the term (via Old French lancier), as well as the rarer or poetic to lance. The term from the 17th century came to refer specifically to spears not thrown, used for thrusting by heavy cavalry, and especially in jousting. A thrusting spear which is used by infantry is usually referred to as a pike.
The Roman cavalry long thrusting spear was not called lance, but contus (from Greek language kontos, barge-pole). It was usually 3 to 4 m long, and grasped with both hands. It was employed by equites contariorum and equites catafractarii, fully armed and armoured cataphracts. Romans weren't the first, though; the first use of the lance in this sense was made by the Sarmatian and Parthian cataphracts from ca. the 3rd century BC, and cavalry long thrusting spear was especially popular among the Hellenistic armies' agema and line cavalry.
The use of the basic cavalry spear is so ancient, and warfare so ubiquitous by the beginning of recorded history, that it is difficult to determine which populations invented the lance and which learned it from their enemies or allies.
The best known usage of military lances was that of the full-gallop closed-ranks and usually wedge-shaped charge of a group of knights with underarm-couched lances, against lines of infantry, archery regiments, defensive embankments, and opposition cavalry. It is commonly believed that this became the dominant European cavalry tactic in the 11th century after the development of the cantled saddle and stirrups (the Great Stirrup Controversy), and of rowel spurs (which enabled better control of the mount). Cavalry thus outfitted and deployed had a tremendous collective force in their charge, and could shatter most contemporary infantry lines. Recent evidence has suggested, however, that the lance charge was effective without the benefit of stirrups.
One of the most effective pre-stirrup lanced cavalry units was Alexander the Great's Companion cavalry, who were successful against both heavy infantry and cavalry units.
The Byzantine cavalry used lance (kontos or kontarion) almost exclusively, often in mixed lancer and mounted archer formations (cursores et defensores). The Byzantines used lance both overarm and underarm, couched.
While it could still be generally classified as a spear, the lance tends to be larger - usually both longer and stouter and thus also considerably heavier, and unsuited for throwing, or for the rapid thrusting, as with an infantry spear. Lances did not have spear tips that (intentionally) broke off or bent, unlike many throwing weapons of the spear/javelin family, and were adapted for mounted combat. They were often equipped with a vamplate, a small circular plate to prevent the hand sliding up the shaft upon impact. Though perhaps most known as one of the foremost military and sporting weapons used by European knights, the use of lances was spread throughout the Old World wherever mounts were available. As a secondary weapon, lancers of the period also bore swords, maces or something else suited to close quarter battle, since the lance was often a one-use-per-engagement weapon; after the initial charge, the weapon was far too long, heavy and slow to be effectively used against opponents in a melee.
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