In a BDSM

BDSM

BDSM is an erotic preference and a form of sexual expression involving the consensual use of restraint, intense sensory stimulation, and fantasy power role-play. The compound acronym BDSM is derived from the terms bondage and discipline , dominance and submission , and sadism and masochism...

 context, a leash or lead is a length of rope, leather, chain, or other flexible material symbolically worn by the submissive

Bottom (BDSM)

In BDSM, a bottom or submissive is the partner in a BDSM relationship or a BDSM scene who takes the passive, receiving, or obedient role, to that of the top or dominant....

 partner. Most leashes attach to collars

Collar (BDSM)

In a BDSM context, a collar is a device of any material placed around the neck of the submissive partner. It is also a term used to show ownership of one partner by another. A person wearing a collar to symbolize their relationship with another is said to be collared...

 or body harnesses, while others loop directly around the person's neck, wrist

Wrist

In human anatomy, the wrist is variously defined as 1) the carpus or carpal bones, the complex of eight bones forming the proximal skeletal segment of the hand;...

, ankle

Ankle

The ankle joint is formed where the foot and the leg meet. The ankle, or talocrural joint, is a synovial hinge joint that connects the distal ends of the tibia and fibula in the lower limb with the proximal end of the talus bone in the foot...

, or sometimes around the male genitals or female body piercings.

The most common human leash has a clip at one end which is attached to a D-ring

D-ring

A D-ring is an item of hardware, usually a metal ring shaped like the letter D. It may be used at the end of a leather or fabric strap, or may be secured to a surface with a metal or fabric strap...

 or loop on a collar. Commonly made of leather

Leather

Leather is a durable and flexible material created via the tanning of putrescible animal rawhide and skin, primarily cattlehide. It can be produced through different manufacturing processes, ranging from cottage industry to heavy industry.-Forms:...

 (occasionally braided for extra flexibility) but sometimes of nylon

Nylon

Nylon is a generic designation for a family of synthetic polymers known generically as polyamides, first produced on February 28, 1935, by Wallace Carothers at DuPont's research facility at the DuPont Experimental Station...

 webbing, small-linked chain or rope

Rope

A rope is a length of fibres, twisted or braided together to improve strength for pulling and connecting. It has tensile strength but is too flexible to provide compressive strength...

. Many people use a strong dog leash

Leash

A leash is a rope or similar material attached to the neck or head of an animal for restraint or control...

, especially a black leather one. Typical length in total is between one and two metres.
Purposes of a leash include:

• More than the wearing of a collar, a leash is a stronger visual indication to people at a BDSM scene
  Scene (BDSM)
  In BDSM, a scene is the stage or setting where BDSM activity takes place, as well as the activity itself. The place where a BDSM activity takes place is usually called a dungeon. A BDSM activity can but need not involve sexual activity or sexual roleplay. A scene will vary, depending on the wishes...
  
   of the slave status of the wearer, to ensure they follow the appropriate BDSM etiquette in relation to the wearer and the dominant partner
• A psychological reinforcement to the submissive of their slave status
• A means whereby the dominant partner can pass cues to the submissive for their physical control and positioning.

The submissive's collar, leash and restraints are largely symbolic of the relationship between the partners, usually indicative of a master-slave relationship

Master/slave (BDSM)

In BDSM, Master/slave or M/s is a relationship in which one individual gives to another ultimate authority over them. It is a form of dominance and submission or total power exchange. The participants may be of any gender or sexual orientation...

. Some submissives experience initial humiliation by a overt public display of their submissive status, but eventually feel more comfortable wearing the leash, which gives them a strong sense of belonging and is regarded as a sign of commitment. Leading a submissive on a leash can give the dominant partner pride and a strong sense of ownership and responsibility.

Leashes are most often used in private or in a secluded public place, or sometimes in public but discreetly so as not to be obvious to others. Sometimes, a leash is used very openly and publicly, especially in a scene club

Scene (BDSM)

In BDSM, a scene is the stage or setting where BDSM activity takes place, as well as the activity itself. The place where a BDSM activity takes place is usually called a dungeon. A BDSM activity can but need not involve sexual activity or sexual roleplay. A scene will vary, depending on the wishes...

 and in animal roleplay.

At a club or play party, D/s players often prefer formal etiquette

Etiquette

Etiquette is a code of behavior that delineates expectations for social behavior according to contemporary conventional norms within a society, social class, or group...

and will use a leash as part of this, to signify that the submissive is not single or, if the leash is held in their mouth, that they are not available for conversation.