Patterns are rarely made with parts meeting at right angles. The junction of the parts is filled in with a fillet to reduce the sharp edge.
Patrick Leach of fame explains fillets to the masses:
"In the old days, leather strips, called fillets where used to accomplish this. You can find little planes designed to make these fillets; the planes are often cast by the patternmakers themselves. The fillets are glued into the pattern and then are shaped with tools that resemble elongated small dumbbells, where the ends of the tools are spheres of different diameters. Today, most patternmakers use bondo and epoxies."
The quote above is from oldtools message 29974

Here are pictures of the dumbbell shaped filletting tools. The set on the right were an ebay acquisition. The seller was in England.
Here is a lifetime supply of leather fillets I also got on ebay!

Fillet Cutter

Here is an unusual fillet cutter (a non ebay acquisition). As shown in the slideshow¹, it is marked PAT APPLIED FOR. It looks a lot like patent 676,728 from June 18, 1901 available at these three sites: I was unable to find this patent until google got into the patent searching game. I'd like to publicly thank them for that! (There is more information on patent searching here and here).

¹Click here to see a slide show of the fillet cutter - created after receiving a request from Norway for more information!

Wax fillets are/were also used. Here's a scan of a box containing 1/32" fillet wax. It's from Adams & Nelson Pattern Shop Supplies and Equipment - now defunct. It's probably from the 1930's or 40's. A time before 5 digit zip codes (Chicago 39) and in the days of telephone exchanges. Their phone number was ALBany 2767.

From Patented Transitional & Metallic Planes in America, 1827-1927 (Vol 1)
by Roger K. Smith

Leather fillets can be seen on the pattern in background and their impession is visible in the casting in the foreground.