North of the Border
The cover of his book shows the skew chisel Ian made from a 1/4" chisel. Details in his book, p. 46.

https://www.highlandwoodworking.com/thecompletedovetail.aspx

But as I said, dovetails, any kinds, can be cut without using any skew or fishtail chisels. Corners can be cleaned with a small chisel, 1/8" if necessary.

Skew or fishtail chisels are nice to have...a point I always stress to beginners so they don't get loaded up with expensive tools before they can even cut any decent dovetails.