A twibill is a double bit adz/axe, i.e., one blade is at a 90* to the handle,
the other is at 0*/180*. This configuration allows ease of use when cutting
out the mortices it was intended, as the operator needn't move to cut out
"horizontal" and "vertical" lines. Twy-bil, Twibble, Twibill, they are all
the same. Eric Sloans' books are a good resource for old tool usage.
I am the proud possesor of 3 twibills...
Henry Roeters