To support the tube (koker) and to give strength to the lower part of the windmill there are "kokerbalken" (girders?). They form a bent together with 2 posts and 2 knee braces.
Taking measurements
Making the mortises
Fitting mortis and tenon. They are aligned on the center of the beams
2 done, 2 to go
Chiseling away
All finished with the tube temporary placed on top in stead of placed in the housing