Let us begin with (1) the motion of a body between two events and (2) two ways of measuring the extent of that motion: *length* and *duration* (or time). The measurement of length and duration is coordinated so that both measures are of the same motion. Length and duration are measured by a rigid rod and a stopwatch, respectively. A smooth manifold of length is called space (or 3D space), and a smooth manifold of duration is called time (or 3D time).

The length and duration of a motion are commonly measured along the trajectory (or arc) of the motion. The length along the trajectory of motion is the *arc length* (or proper length or simply *length*). The duration along this trajectory is the *arc time* (or proper time or simply *time*).

Once the length and duration are in hand, the next step is to form their ratio. The ratio with arc time as the independent variable and arc length as the dependent variable is the *speed*. Which is to say, speed is the time rate of arc length change.

Note that the ratio could just as well be formed in the opposite way, with the arc length as the independent variable and the arc time as the dependent variable. This ratio is called the *pace* from its use in racing, in which an arc length is first set and then the racer’s arc time is measured. As another way to state this, pace is the space rate of arc time change.

The reference trajectory for measuring the length of a motion is the minimum length trajectory between two event points. The length along this trajectory is the *distance* between the two event points, which forms the metric of space. Distance is represented as a straight line on a length-scale map.

The reference trajectory for measuring the duration of a motion is the minimum time trajectory between two event instants. The duration along this trajectory is the *distime* between the two event instants, which forms the metric of time. On a map, two isochrons are separated by a constant distime. Distime is represented as a straight line on an time-scale map.

Motion has direction as well as extent, and direction may also be measured in two ways. Consider the motion of rotation, which can be measured as a proportion of a circle and as a proportion of a cycle. For example, in an analogue clock a minute hand that moves the length of a right angle correspondingly moves a duration of 15 minutes and *vice versa*. The direction of motion may be measured by either length or duration.

A motion measured with length direction and distance comprises a vector *displacement*. A motion measured with time direction and distime comprises a vector *dischronment*. The ratio with time as the independent variable and displacement as the dependent variable is called the *velocity*. The magnitude of the velocity vector is the *speed*. The ratio with distance as the independent variable and dischronment as the dependent variable may be called the *legerity*. The magnitude of the legerity vector is the *pace*.

There are three dimensions of motion, and correspondingly three dimensions of length and duration. The three dimensions of length comprise *space*. The three dimensions of duration comprise *time*. The three dimensions of time come as a surprise, since the distime is often a parameter for ordering events. But the scalar distime should not be confused with the vector dischronment, which has three dimensions of motion measured by duration.