3 Chapter 1 Introduction to Dimensioning and Tolerancing Copyright Goodheart-Willcox Co., Inc. Development of a Distance Measurement Standard It would be impossible to produce tools or machines that build and verify accurate distances without an accurate distance measurement standard. Distance measurement standards that are in place today were developed over a few thousand years. Units of measurement such as cubits, spans, palms, and digits were in use 5000 years ago. Those early measurement standards were based on defi n- ing features present in nature. The defi ning features varied, causing the standards to be based on variable distances rather than fi xed distances. As an example, a cubit was the length of a person’s forearm from the elbow to the tip of the middle fi nger. This evolved to a permanent standard referred to as the Royal Cubit. The Royal Cubit standard was used by the Egyptians. It was made of black granite and its length (20.67″) was based on the Pharaoh’s arm. This black granite standard was used as a reference against which mea- suring sticks were calibrated. It was subdivided into small increments to allow relatively short distances to be measured. Many other distance measurement standards became established and were used throughout the world. Gradually, standards improved over time to allow for greater measurement precision and accu- racy. In 1799, the standard meter was established. It was based on a distance equal to one ten-millionth of the meridional distance from the North Pole to the equator on a line passing through Paris, France. However, this distance was very diffi cult to ver- ify. In 1960, there was an international agreement to establish the meter equal to 1,650,763.73 wave- lengths, in a vacuum, of the radiation emitted by a specifi c form of krypton. In 1975, an international agreement was reached to establish the meter equal to the distance traveled by light in a vacuum during 1/299,792,458 of a second. In the US Customary sys- tem, the length of an inch is referenced to the stan- dard meter. An inch is exactly equal to 25.4 mm. The existence of an accurate distance measure- ment standard makes it possible to produce and measure with very precise accuracy. Just as measure- ment standards have improved in accuracy, so have the tools used to measure lengths and other distances precisely. See Figure 1-1. The fi gure shows two Scale and outside caliper Dial caliper Goodheart-Willcox Publisher Figure 1-1. A simple caliper and scale are shown on the left. As distance measurement standards evolved, so did the sophistication of measurement tools, such as the dial caliper shown on the right.