Lead

Leads (IPA: /liːd/) also known as leashes, are items used to leash and lead passive mobs.

Usage and behavior
Right-clicking on a mob while holding a lead will tie it to the mob, allowing it to be moved by the player. Right-clicking on the mob again will release it. Other inventory spaces can be selected while holding the lead; multiple mobs can be attached to a lead at once, but require one lead per mob led.

With a mob on a lead, right-clicking onto a fence will attach any leads you are holding to the fence with a visible knot. Attacking the knot or removing the fence post removes the lead(s). Also if an arrow (including those shot by skeletons) hits the knot, then the knot will break and drop as an item (even in creative mode).

A lead will stretch a maximum of ten meters before breaking. Whenever a lead is removed or broken it will drop as an item at the location of the mob.

Most passive mobs can be "leashed". These include: horses, wolves, cats, sheep, pigs, cows, mooshrooms, chickens, and squids. The same can be done to Iron Golems and Snow Golems. Although Zombie Pigmen could be considered passive until provoked, they cannot be attached to a lead. Villagers can not be leashed since the 1.6.2 update. Before patch 1.6.2, villagers could only be leashed in multiplayer, as it required one player to open the trade interface and another to leash the villager while the trade interface is open, as two players cannot access the same trade interface at the same time (verified). Bats are the only passive mobs which cannot be attached to a lead, and any mobs which naturally despawn (such as the squid) will still despawn if they are attached, unless the mob was named with a Name Tag.

You can use the lead to guide horses across deep water. This is useful, as you will be thrown off the horse if you enter water deeper than two blocks.

It is not possible to attach hostile mobs by traditional methods, but can be done by modifying the region save files. Certain mobs (e.g. bats) however, as well as players, cannot be attached even with this method.

Mobs can be suspended from leads if the fence they are attached to is 7 blocks above the ground. This factor goes for both one and two block tall mobs. The method of suspending mobs is an alternative to adding fences to prevent a mob from moving while on a lead. However, once suspended, the likelihood of a mob dying from fall damage is high, especially if the fence is only the minimum of 7 blocks above the surface.
 * This can be seen here http://www.youtube.com/watch?v=b4jqUuyS844

If the player is holding a lead connected to a mob, walks through a nether portal, then walks back through it, the player will still have the leads attached to the mob. However, if the mob walks into a nether portal, the leads are broken and dropped as items in the other dimension.

Trivia

 * A lead can extend out to 5 blocks when tied between a fence post and a mob.
 * Leads can also be attached to a Nether Brick Fence.
 * The lead is named as 'leash' in the texture file.
 * Untamed ocelots can be captured and tied up using a lead.
 * In Creative mode, players can leash mobs, pull the mob into the air and tie it to a fence post up high. However, the lead tends to break if the player moves upward too quickly.
 * Leashing farm-able mobs (e.g. cows, pigs) seems to negate the function of wheat (for cows) or carrots (for pigs) after releasing the mob.
 * Leads cannot be attached to hostile mobs, players, villagers, or bats.
 * You can ride in a boat while leading a mob on a leash. The mobs move as fast as the boat does and the leads should not break.
 * Through NBT editing you can leash all mobs, including villagers, creepers, etc.
 * Also through NBT editing, you can leash mobs to other mobs.
 * You can destroy a knot tied on a fence by shooting it with a bow and arrow.
 * Landing a leashed mob (that has recently been suspended in air) in water will negate the fall damage.
 * You cannot lead a villager, as the right click will just open up the trading menu.

Gallery
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