# Definition:Linear Measure

## Definition

**Linear measure** is the means of measurement of physical displacement.

### Symbol

The usual symbol used to denote **linear measure** is $l$.

### Dimension

**Linear measure** is one of the fundamental dimensions of physics.

In dimensional analysis it is assigned the symbol $\mathsf L$.

### Units

The SI unit of **linear measure** is the **metre**.

The CGS unit of **linear measure** is the **centimetre**.

The FPS unit of **linear measure** is the **foot**.

### Length

**Length** is linear measure taken in a particular direction.

Usually, in multi-dimensional figures, the dimension in which the linear measure is greatest is referred to as **length**.

It is the most widely used term for linear measure, as it is the standard term used when only one dimension is under consideration.

**Length** is the fundamental notion of Euclidean geometry, never defined but regarded as an intuitive concept at the basis of every geometrical theorem.

### Breadth

**Breadth** is linear measure in a dimension perpendicular to length.

In the context of a two-dimensional geometric figure, the **breadth** is in the plane of that figure.

In a three-dimensional figure, the choice of which direction is referred to as **breadth** is often arbitrary.

### Depth

**Depth** is linear measure in a dimension perpendicular to both length and breadth.

The choice of **depth** is often arbitrary, although in two-dimensional diagrams of three-dimensional figures, **depth** is usually imagined as being the dimension perpendicular to the plane the figure is drawn in.

### Height

**Height**, like depth, is used as a term for linear measure in a dimension perpendicular to both length and breadth.

However, whereas depth has connotations of **down**, **height** is used for distances **up** from the plane.

### Thickness

**Thickness**, like breadth, is used as a term for linear measure in a dimension perpendicular to both length and depth.

However, whereas breadth has connotations of **across**, **thickness** is used for distances **through** the solid figure.

### Distance

The **distance** between two points $A$ and $B$ in space is defined as the length of a straight line that would be drawn from $A$ to $B$.

## Also see

- Definition:Distance in the context of analysis

## Sources

- 1969: J.C. Anderson, D.M. Hum, B.G. Neal and J.H. Whitelaw:
*Data and Formulae for Engineering Students*(2nd ed.) ... (previous) ... (next): $1.$ Units and Abbreviations: $1.2$ SI units $(1)$ Basic units - 1976: Ralph J. Smith:
*Circuits, Devices and Systems*(3rd ed.) ... (previous) ... (next): Chapter $1$: Electrical Quantities: Definitions and Laws: The International System of Units: Table $1$-$1$ Basic Quantities