Units

Abstract

It is helpful to pick the right set of units in order to reason easily about a physical subject. In different contexts, different units are appropriate. Small boat enthusiasts will recognize the need to determine whether depth on a chart is labeled in feet or fathoms. It is common in the United States coastal waters to label the depths in feet where mostly small boats will be expected to be found. But commercial vessels might prefer to think in fathoms (a fathom is six feet) since their depth requirements will be some number of fathoms (and thus a much larger number of feet). The phrase “mark twain” was used on riverboats for which twelve feet of water provided safe passage.