Why is it that a lower number means a heavier canvas tarp cloth? The
explanation has to do with the weight of a square yard of cloth and the
number 19. If a yard of cloth weighs 11 ounces, it is #8 canvas cloth (19
minus 11 equals 8). If a yard of cloth weighs 18 ounces, it is #1 canvas
cloth. #8 is a fairly lightweight canvas, and #1 is heavy canvas.
Why 19? Your guess is as good as mine.
What happens if a square yard weighs 19 ounces or more? There's an
extended system for measuring the extra heavy duty canvas tarp material,
called "naught duck" measurement. At 19 ounces per yard, it is 1/0 naught
duck cloth, at 20 ounces, it is 2/0 naught duck, etc.
I have also seen reference to measuring the weight of a 36" by 22" square of
cloth when calculating numbered duck or naught duck canvas cloth weight. It
seems that duck canvas is more densely woven than ordinary canvas, or maybe
the information was incomplete.
Are you sorry you asked about canvas cloth weight measurement yet? Canvas cloth may or may not be canvas duck cloth. It might be single fill or double fill canvas duck, and it might be measured by the weight of
a square yard, or the weight of almost a square yard. That weight might
be used directly as the unit of measurement, or it might be subtracted from
the number 19, or it might go into a whole different measurement system if
the weight of a square yard (or almost a square yard) is greater than or
equal to 19 ounces.