Length and units
How long is it, in which unit, and how do you say the same length in a different unit?
Where you are 7 inside
What this area is about
A length is the number of equal units that fit along a thing, end to end with no gaps. This area turns that into reading a ruler, names the standard units people agree on, and changes a length from one unit to another without changing the length itself.
How these fit together
The procedure. Measuring length reads a ruler: a row of units already laid out, with the counting printed on it.
The units. Customary length units (inch, foot, yard, mile) and metric length units (millimeter, centimeter, meter, kilometer) give each unit a fixed size and a sense of what it fits.
Reading more finely. Measuring to the half and quarter inch reads an end that falls between inch marks. Length on a number line finds the distance between two points when neither sits at 0: subtract.
Conversions. Customary conversions multiply or divide by 12, 3, 5{,}280 and the like. Metric conversions multiply or divide by a power of ten. Both cover weight and liquid units too, not only length.
A path through it
Measuring length comes first; everything else here builds on it. Then the two unit families and the number line. Half and quarter inches follow customary units and need fractions. Customary conversions need multiplication facts. Metric conversions wait for metric prefixes.
Next: perimeter, segments and rays and conversion factors.