I just finished mortising the first of two breadboard ends for a table I am
making.  The ends are 2" wide x 1" thick red oak, and I decided to try
cutting the 3/8" mortise by hand.  My concern was in keeping the sides of the
mortise straight and square for 18".  I ended up clamping the 2" high end
board to a 6" high board with one of my antique wooden hand screws (1st OT
content).  Then I clamped the clamp to the workbench so that the boards
wouldn't go anywheres.  I took my 3/8" mortise chisel and taped a 5/16" thick
block of wood about 3" up on the chisel shaft.  I had already marked out the
mortise with my marking gage, so now all I had to do was line up the end of
the chisel on the layout lines and keep the block of wood taped to the chisel
tight against the 6" 'fence'.  It worked fairly well.

I cut most of the tenon on the table...er...tailed aprentise, but I left it a
little thick.  I trimmed it down with my skew rabbet woodie (2nd OT content).
 I bought that plane on a whim, and it just keeps coming in handy.  Anyways,
I've got a nice snug fit between the mortise and tenon that I am very happy
with.

Now that I figured out a way to do it, I would like to hear how others do it.
 BTW, it's a lot of work to cut an 18" long mortise in red oak 3/4" to 7/8"
deep.

David