Lead

Leads, also known as leashes, are items used to leash and lead mobs.

Usage and behavior
Right-clicking on an animal with a lead in your hand will tie it to the animal, allowing you to lead that animal; right-clicking on the animal again will release it. You can use other inventory items while holding the lead, and you can lead multiple animals at once.

Once you have an animal on a lead, right-clicking onto a fence will attach any leads you are holding to the fence with a visible knot. NOTE: This will only work if you are currently holding an additional leash in your hand when tying the knot. Attacking the knot or removing the fence post removes the lead(s).

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 animal; it will not automatically return to your inventory.

Most passive mobs can be "leashed". These include: horses, wolves, cats, sheep, pigs, cows, mooshrooms, chickens, squid, Iron Golems, and Snow Golems. Bats and Villagers 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.

Note that it is technically possible to attach hostile mobs to a lead, but only by modifying your region save files. Certain mobs (e.g. bats) however, as well as players, cannot be attached, even with this method.

Leashing Iron Golems makes them very effective bodyguards as they can keep up with the player even whilst sprinting, although negatively they may run into difficulty navigating forestry areas and caves which could cause the lead to break or because of their proximity to the player during combat the player could damage an Iron Golem.

Trivia

 * The lead can extend out to 5 blocks when tied between a fence post and a mob.
 * They are 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 it into the air and tie it to a fence post up high (see images below). However, the lead tends to break unless you move upwards slowly. (Gravity pulling the object downwards and the lead pulling it upwards act against each other and makes the lead behave like a rubber band.)
 * Multiple mobs can be tied to the same fence post.
 * You can use the lead to capture several mobs at the same time and walk them around altogether.
 * Despite that it's made of string, it looks like that is made of rope.
 * Slimeballs come from hostile mobs, making the lead uncraftable in peaceful mode, unless you already have a slimeball in your inventory.
 * If a lead is broken while attached to a fence post it may sometimes still show the knot until clicked again.
 * This can only be achieved with a horse. Other mobs avoid the lead's limit. A pig will be pulled back repeatedly if ridden and controlled with a carrot on a stick.

Gallery
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