You can use any unit you like when you're measuring the length
of the rope.
But if you ever want to write it down, memorize it, or tell other people
about it, then you'll want to convert your measurement into the unit
that gives you the most convenient number.
-- If it's a short rope, like one that just barely reaches across your bedroom,
then you'd want to describe its length in either centimeters or meters.
-- If it's a longer rope, like one that's coiled up on a reel and if you unwound
it then it could reach from your house to the main post office downtown, then
meters is probably the best unit to use.
-- If it's a rope on a reel that can reach from your house to the White House
in Washington when it's unwound, then you'd want to describe its length in
kilometers.
