A guige is a long strap

Strap

A strap, sometimes also called strop, is an elongated flap or ribbon, usually of fabric or leather.Thin straps are used as part of clothing or baggage, or bedding such as a sleeping bag. See for example spaghetti strap, shoulder strap...

, typically 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:...

, used to hang a shield

Shield

A shield is a type of personal armor, meant to intercept attacks, either by stopping projectiles such as arrows or redirecting a hit from a sword, mace or battle axe to the side of the shield-bearer....

 on the shoulder or neck. This technique was primarily done when the shield was not in use. Nevertheless, soldiers also wore the strap in this fashion in combat: it allowed the two-arm combat: to handle a second weapon, to allow the shield be worn on the back while using a two-handed sword

Two-handed sword

A two-handed sword, used as a general term, is any large sword designed to be used primarily with two hands:* the European longsword, popular in the Late Middle Ages and Renaissance....

, to use a spear

Spear

A spear is a pole weapon consisting of a shaft, usually of wood, with a pointed head.The head may be simply the sharpened end of the shaft itself, as is the case with bamboo spears, or it may be made of a more durable material fastened to the shaft, such as flint, obsidian, iron, steel or...

 with two hands or various siege

Siege

A siege is a military blockade of a city or fortress with the intent of conquering by attrition or assault. The term derives from sedere, Latin for "to sit". Generally speaking, siege warfare is a form of constant, low intensity conflict characterized by one party holding a strong, static...

 machines without discarding the shield, etc.
Some guiges had buckles to adjust its length. They could be attached to the shield in different ways: in the upper part, upside down, diagonally, or diametrally.

Most information about the usage of guiges comes from various Mediaeval works of art, such as the Bayeux Tapestry

Bayeux Tapestry

The Bayeux Tapestry is an embroidered cloth—not an actual tapestry—nearly long, which depicts the events leading up to the Norman conquest of England concerning William, Duke of Normandy and Harold, Earl of Wessex, later King of England, and culminating in the Battle of Hastings...

.