Oak tag, also known as tagboard or manila paper, is commonly
used in commercial settings as the pattern paper of choice --
none of this flimsy mimsy stuff you get in pattern envelopes
for home sewing. As it is thick (think cardstock), it's durable and
is therefore easily used for cutting a pattern multiple times.
As I mentioned earlier, oak tag patterns are traced onto fabric
(and removed) before cutting happens, whereas patterns on
tissue paper are usually pinned on and cut around directly. This
tracing method keeps the pattern accurate by protecting it from
scissor nicks, wiggly lines caused by pin creases, and the like.
Oak tag is expensive paper, though, and hard to find.
It is often sold on the roll (for $300!) or by the yard (still
expensive at $4.75 a yard). However, I recently discovered
150 lb. manila board at art supply retailers for much less.
It comes in sheets from 9"x12" to 22"x30". This may not
be particularly useful for large items of clothing, like pants or
dresses, but I suspect it will work just fine for bagmaking.
All that said, I just received my first box of oak tag from
Dick Blick. At $21.50 for 100 sheets, it's a steal!