Lance

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Name	lance
Appearance	lance
Damage vs. small	1d6
Damage vs. large	1d8
To-hit bonus	+0
Weapon skill	lance
Size	one-handed
Base price	10 zm (+10/positive enchant)
Weight	180
Material	iron

A lance is a weapon in NetHack. It is meant to be used while mounted and gives a chance dependent on your lance skill (with each higher rank giving an extra +20%) of jousting a monster on a successful attack, granting an additional 2d10 damage and causing the victim to be pushed back a square if they are not already dead. Very heavy monsters (eg mastodons) and monsters who have a wall or another monster behind them will not be pushed back. The same thing happens if entering the square would kill the monster (eg a lava pool).

Jousting will also stun the victim. Stunning monsters seems to reduce the frequency of their attacks, or at least make them attack less. It also seems to hamper their use of special attacks (e.g. breath weapons) and teleport. Jousting can cause the lance to break; as lances are difficult to come by, it will be hard for you to find a replacement should yours break. High luck and a positively enchanted lance reduce the chance of breakage.

A lance can also be applied to attack two squares away, or any of the 8 squares that are a knight's jump away. However, there is a to-hit penalty for applying the lance when the wielder is at basic skill level or lower. It is possible to stay a knight's jump away from monsters who are able to throw missiles and use spells and breathweapons to attack them with the lance without them being able to retaliate.

A deadly jousting tactic is to let the monster close, then apply the lance on it until it moves adjacent, then hit it normally. Since at high skill levels a joust is nearly guaranteed, the monster will be pushed back for another round of lance thrusts, then will move adjacent again only to be jousted and so on until the monster is dead, usually without ever having an opportunity to attack.

Lance skill

Lance
Max	Role
Basic	Priest, Tourist
Skilled	Samurai, Valkyrie
Expert	Knight

The lance is the only weapon to use lance skill, and there are no artifact lances.

Encyclopedia entry

With this the wind increased, and the mill sails began to turn 
about; which Don Quixote espying, said, 'Although thou movest
more arms than the giant Briareus thou shalt stoop to me.'
And, after saying this, and commending himself most devoutly
to his Lady Dulcinea, desiring her to succor him in that trance,
covering himself well with his buckler, and setting his lance
on his rest, he spurred on Rozinante, and encountered with the
first mill that was before him, and, striking his lance into
the sail, the wind swung it about with such fury, that it broke
his lance into shivers, carrying him and his horse after it,
and finally tumbled him a good way off from it on the field in 
evil plight.
        [ Don Quixote of La Mancha by Miquel de
          Cervantes Saavedra ]