Amicus, its two different things, generally its the feather clamp that determines whether it is helical or not and a setting on the nock holder for, 120-3 fletch, 90-4 or 105/75-4 fletch. My first fletching jig only had the 120-3 setting, so about 40 years ago I experimented with four fletch by fletching the two hens feathers and then flipping the arrow over and doing two more, so I actually ended up with a 120/60-four fletch! One thing I prefer about the 105/15 to the 90 degree, is fitting them into an arrow box or bow quiver, I find that it is easier to get them to fit without the fletching hitting each other.