A combination square is a tool

Tool

A tool is a device that can be used to produce an item or achieve a task, but that is not consumed in the process. Informally the word is also used to describe a procedure or process with a specific purpose. Tools that are used in particular fields or activities may have different designations such...

 used for multiple purposes in woodworking

Woodworking

Woodworking is the process of building, making or carving something using wood.-History:Along with stone, mud, and animal parts, wood was one of the first materials worked by early humans. Microwear analysis of the Mousterian stone tools used by the Neanderthals show that many were used to work wood...

, stonemasonry

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...

 and metalworking

Metalworking

Metalworking is the process of working with metals to create individual parts, assemblies, or large scale structures. The term covers a wide range of work from large ships and bridges to precise engine parts and delicate jewelry. It therefore includes a correspondingly wide range of skills,...

. It is composed of a ruled

Ruler

A ruler, sometimes called a rule or line gauge, is an instrument used in geometry, technical drawing, printing and engineering/building to measure distances and/or to rule straight lines...

 blade and one or more interchangeable heads that may be affixed to it. The most common head is the standard or square head which is used to lay out or check right

Right angle

In geometry and trigonometry, a right angle is an angle that bisects the angle formed by two halves of a straight line. More precisely, if a ray is placed so that its endpoint is on a line and the adjacent angles are equal, then they are right angles...

 and 45° angles.

Uses

• Measuring angles — A combination square can reliably measure 90° and 45° angles. The 45° angle is used commonly in creating miter joint
  Miter joint
  A miter joint , sometimes shortened to miter, is a joint made by bevelling each of two parts to be joined, usually at a 45° angle, to form a corner, usually a 90° angle...
  
  s.
• Determining flatness — When working with wood the first step is to designate a reference surface
  Surface plate
  A surface plate is a solid, flat plate used as the main horizontal reference plane for precision inspection, marking out , and tooling setup. The surface plate is often used as the baseline for all measurements to the workpiece, therefore one primary surface is finished extremely flat with...
  
   on a board which is known as the face side. The rest of the workpiece is measured from the face side with the next surface being the face edge.
• Measuring the center of a circular bar or dowel
  Dowel
  A dowel is a solid cylindrical rod, usually made of wood, plastic or metal. In its original manufactured form, dowel is called dowel rod.Dowel rod is employed in numerous, diverse applications. It is used to form axles in toys, as detents on gymnastics grips, as knitting needles, as structural...
  
  . The rule is assembled through the center of the center square, the two cast iron legs of the center square are then placed against the outside of the bar (dowel) allowing a center line to be scribed alongside the ruler. Perform this action at two locations and the intersecting lines will approximate the center of the bar (dowel).
• Protractor
  Protractor
  In geometry, a protractor is a circular or semicircular tool for measuring an angle or a circle. The units of measurement utilized are usually degrees.Some protractors are simple half-discs; these have existed since ancient times...
  
   head allows angles to be set and measured between the base and ruler.
• A rudimentary level for approximating level surfaces is incorporated in the protractor and also the 45° holder.
• Marking the work surface; with the included Scribe Point stored in a drilled hole in the Square Base.

In woodworking, the starting raw material is neither flat nor square, however, the end product such as a table must be flat and have corners and legs which are square.

In metalworking, it is useful for a wide variety of layout and setup tasks. When used correctly, a fairly high degree of precision can be achieved. One use would be setting large items at the required angle in machine vices, where the long reach of the ruler and firm, heavy base aid the process.

Invented by Laroy S. Starrett in 1878, the combination square continues to be a commonplace tool in home workshops, construction jobsites and metalworking