Barbara,

    I'm not really sure how to post this back to SAH-L, but I'm pretty sure
one of the Pinheads will get it there.  While I'm not really researched in
the field, clapboard has been in the new world as long as the Europeans have.
 There is evidence for it's use at the Plymouth colony, dating as early as
1620.  As for the shape, tapered boards far predate parallel sided boards in
general usage, since they are much more economical to produce.  The boards
are riven, or split out of the round log.  The process is to split a section
of clear log in half, then split the halves into quarters and the quarters
into eighths.  At this point, the eighths are split with a froe.  Depending
on the diameter of the original log, the resulting boards produced can be as
wide as 12" or more with a thickness at the wide edge varying between 1/2"
and 1" tapering to a knife edge toward the center of the tree.  These boards
can then be dressed with a drawknife to a consistent thickness and exposure
as desired.  In the grand scheme, this is far more efficient than sawing
boards until the introduction of relatively fast sawmills in the 19th C.
Been there, done that.  As far as when it became fashionable to reproduce the
natural shape while wasting about 40% of the tree in the form of sawdust, I
would suspect that followed the advent of fast sawmills in the mid-1800's.
Water driven sash mills-using a single or ganged blades in a frame which
moved up and down were not very fast and were generally suited to cut
straight and square sided lumber.  Cutting a tapered board requires one or
more of the following: changing the angle of the cant each time it passes
through the saw blade, changing the angle of the blade each time the cant
passes through it, or having multiple blades at slightly different angles
operating at the same time as the cant passes through.  Any of these
conditions would be predicated on the ability to do them quickly to make a
profit.  Enough of my pedantic ramblings.  I think the answer lies indirectly
in the advancement of sawmill technology.

David J. Dauerty