It is fairly easy to confuse the physical dimensions of a quantity with the units used to measure the dimension. We usually consider quantities like mass, length, time, and perhaps charge and temperature, as fundamental dimensions. We then express the dimensions of other quantities like speed, which is length/time, in terms of the basic set. The point is that every quantity which is not explicitly dimensionless, like a pure number, has characteristic dimensions which are not affected by the way we measure it. As we will see later, this provides a useful check on any calculations we do. Units give the magnitude of some dimension relative to an arbitrary standard. For example, when we say that a person is six feet tall, we mean that person is six times as long as an object whose length is defined to be one foot. The standard size chosen is, of course, entirely arbitrary, but becomes very useful for comparing measurements made in different places and times. Several national laboratories are devoted to maintaining sets of standards, and using them to calibrate instruments.