Twybil

Twybils are made in a range of sizes, depending on the size of the work they're intended for. The length of the side handle remains roughly constant at around a hand's span and so their proportions change. A small one intended for making hurdles^[8][9] may have three arms of approximately equal length, a large framing twybil may be three feet long and yet still have the same handle. Diderot's Encyclopédie of the 18th century shows a bisaiguë in use for paring end grain of a large joint, much as a modern slick chisel, which is long enough to rest on the carpenter's shoulder.^[10]

The shape of the working edges varies with size and purpose. Large twybils have short straight edges for splitting, like an axe or tomahawk. Small ones have a more curved edge, curving back beneath to a shape that is more like a short knife or oyster knife, another tool used with a similar splitting and prying action. Both styles are forged and sharpened symmetrically. The levering end is asymmetrical, with a single bevel as for a chisel. Large framing twybils are straight, with this bevel on the outside. Small ones are curved or hooked,^[3] with the bevel inside.

Most surviving examples are old enough that they are forged from iron rather than steel, with hard steel edges welded to them. Modern examples may be wholly of steel, frequently recycled truck leaf springs. A socket is forged on the side for the handle, this being a short wooden handle, often made to the user's own preference. Twybils are rare today, even amongst the recreators of specialist green woodworking tools. As such, good usable examples may command high prices. Ashley Iles is one of the few bulk production manufacturers to offer one.^[11]

The mortise chisel, even in its heavyweight "pigsticker" form, is used differently from a twybill, although the two may be used together. The twybill cuts the sides of mortices, along the grain. Its action is a splitting and prying one, so only requires a handle for leverage and is never struck. Mortise chisels are used for heavy chopping across the grain, are nearly always struck, and are used to square up the ends of square-ended mortises. Both tools are used for levering out chunks when first clearing out a mortise and so have similarly shaped bevels, often with a curved bevel surface for a better fulcrum action.

Slicks are other specialised chisels also used to work on the sides of mortises, but are used for final clean-up to make an accurate and smooth-sided mortise, after the rough chopping has been carried out.

In the heyday of the twybil, mortises in small work were often round-ended and so could be cut very quickly by brace and twybil alone, the tenon being rounded to fit.^[9] Only large, or high-quality work required the square ends and smoothed sides of a precise mortise, trimmed by this variety of chisels.

The apprentice will often use all three mortising tools interchangeably and randomly, making much effort of removing the waste as small chips. The skilled framer uses each appropriately in turn, working faster, with less effort and not bothering to tear a large block of waste into fragments. They are also less likely to damage a precise edge by levering with a sharp, brittle chisel edge.