Dear Ralph,

Thank you for your kind reply.  I am familiar with the type of fence you
describe, sheltered easterner though I may be.  There is a horse farm
right outside my window with just such a fence.  My problem was not with
the type of fence, but the apparent inconsistency between Twain's
description of the topmost plank and every illustration of the
whitewashing scene that ignores it.  There is also a matter of the
dimensions which Twain states as being “thirty yards of board fence nine
feet high.  This would hardly be the picturesque fence used for
horses.  A corral fence with those dimensions would be more suited to
enclose a giraffe, and  Aunt  Polly didn't own one.

At any rate, the mystery of the fence was solved for me by Barbara
Schmidt and Victor Fischer who provided me with the following
information.  I am taking the liberty to pass it on to you.

Twain's original fence was just the type of fence you described, but
later exaggerated  to the dimensions cited above.  True Williams who
illustrated the first edition worked from the original manuscript and
drew Tom whitewashing a four foot high plank fence.  Since then,
apparently, most illustrations have shown the vertical fencing implied
by the dimensions.

Thanks again,

John