A mason's mark is a symbol

Symbol

A symbol is something which represents an idea, a physical entity or a process but is distinct from it. The purpose of a symbol is to communicate meaning. For example, a red octagon may be a symbol for "STOP". On a map, a picture of a tent might represent a campsite. Numerals are symbols for...

 often found on dressed stone

Rock (geology)

In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

 in buildings and other public structures.

In stonemasonry

Scottish rules issued in 1598 stated that on admission to the guild

Guild

A guild is an association of craftsmen in a particular trade. The earliest types of guild were formed as confraternities of workers. They were organized in a manner something between a trade union, a cartel, and a secret society...

, every mason

Masonry

Masonry is the building of structures from individual units laid in and bound together by mortar; the term masonry can also refer to the units themselves. The common materials of masonry construction are brick, stone, marble, granite, travertine, limestone; concrete block, glass block, stucco, and...

 had to enter his name and his mark in a register.

There are two types of marks used by stonemasons

Stonemasonry

The craft of stonemasonry has existed since the dawn of civilization - creating buildings, structures, and sculpture using stone from the earth. These materials have been used to construct many of the long-lasting, ancient monuments, artifacts, cathedrals, and cities in a wide variety of cultures...

.

1. Layout marks, which delineated the position of a piece of stone in the overall design. Since stone was usually worked at the quarry
   Quarry
   A quarry is a type of open-pit mine from which rock or minerals are extracted. Quarries are generally used for extracting building materials, such as dimension stone, construction aggregate, riprap, sand, and gravel. They are often collocated with concrete and asphalt plants due to the requirement...
   
    or in the masons' yard, rather than in situ, such marks were necessary for efficient operations. Medieval carpenters
   Carpentry
   A carpenter is a skilled craftsperson who works with timber to construct, install and maintain buildings, furniture, and other objects. The work, known as carpentry, may involve manual labor and work outdoors....
   
    also used this sort of mark.
2. "Signature" marks that pertained to a particular mason or workshop. This is what is generally meant by the term. Merchants also had such marks, used to mark their goods.

The exact purpose of mason's marks is unclear, although it is generally assumed that they mark the working of a piece of masonry by a particular mason, in order to claim payment. Others are assumed to indicate the position in which a stone should be laid. It has also been suggested that marks indicate the origin of the stone, or the location in which it was worked.

In Freemasonry

Freemasonry

Freemasonry

Freemasonry is a fraternal organisation that arose from obscure origins in the late 16th to early 17th century. Freemasonry now exists in various forms all over the world, with a membership estimated at around six million, including approximately 150,000 under the jurisdictions of the Grand Lodge...

, a fraternal order that uses an analogy to stonemasonry for much of its structure, also makes use of marks. A Freemason who takes the Mark Master Degree will be asked to create his own Mason's Mark, as a type of unique signature or identifying badge. Some of these can be quite elaborate.