JacksonD wrote:
....
If biscuits add no strength, and are solely for alignment, then why do
we glue them in?

They only replace wood which was removed to insert them; how can they
add significant strength over the solid wood which was there previously?

I didn't say "no strength" earlier, I said "mostly for alignment".

It also depends on how you define and test the joint strength and how
the joint is made. In a well-fitting edge joint, the glue bond will be
as strong or stronger than the wood, anyway, in bending. To see this,
make a few and then break them by bending--almost all will fail away
from the joint, not at the joint itself (again, assuming a well-fitting
joint and adequate clamping pressure).

If, otoh, the joint doesn't fit particularly well or isn't clamped
properly, then the biscuit may be essentially the only thing actually
holding the two pieces together.

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