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The sides of the chisel form a slight trapezoid, so that if your chisel isn't perfectly aligned with the cut you won't damage the sides of the mortise, and more importantly, there is a lot less of a chance for the chisel to get stuck. You just push the chisel in the mortise a little and it loosens up. Lighter sash mortise chisels are ground parallel but that's because they are designed for shallower mortises in window sashes which are usually in soft wood. Some manufacturers say that that parallel sides make it easier to guide and align the chisel with the mortise but in fact the alignment of the mortise is determined by the first stroke of the blade into the wood, long before the sides of the chisel can have any effect.
I assume that most mortise chisels are not shaped like this, but I don't really know that; just be aware of it. Depending on how you sharpen this may be good or bad; I am thinking about side clamping. Lie Nielsen does not mention what they do, but Lee Valley claims that they do. I have not tried the Lee Valley mortise chisels, but I really like the other chisels I have from them.
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The chisel sides are slightly relieved to reduce sidewall friction and allow for minor cut corrections. Each has a 25° primary bevel to make deep cuts easier, with a 35° secondary bevel to give them a durable cutting edge. To aid in levering out waste, the heel of the bevel is rounded to provide a smooth fulcrum. A2 or PM-V11 tool steel versions are available.
Lee Valley does not offer a 3/4" version. Then again, I do not see this from Lie Nielsen either. Sorby Registered Mortise Chisels do come in 3/4"