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**I also occasionally host live design sessions where you can join and ask questions.
A source of confusion for beginners is the fillet tool. This is because there are two types of fillets but they share the same name. You have Sketch Fillet which works by applying rounded edges to 2D shapes. This type of fillet essentially places an arc at the intersection of two lines and can be found under the Modify menu while in 2D sketching mode.
The other type of fillet is the 3D Fillet tool. This type of fillet also applies a rounded curve to sharp edges but it works on 3D shapes as opposed to 2D sketches.
The confusion occurs because the 3D fillet tool can be called with the shortcut "F". The problem is that while in 2D Sketch mode, if you press the keyboard shortcut "F", Fusion will automatically kick you out of 2D Sketch mode and into 3D mode. Basically the same thing you get when you click on "Finish Sketch". That's because Fusion thinks you are trying to access to the 3D Fillet tool.
The "F" shortcut does not apply to 2D Fillet. Therefore, if while in 2D Sketching mode and you hit the "F" key and try to select a sketch line, nothing happens. You'll see that you won't be able to select the line. But if you select the edge of a 3D shape, you'll see that you'll be able to fillet that edge.
Hopefully in a future update Fusion will make the "F" shortcut contextual so that if you are in 2D Sketch mode it will call the 2D fillet tool and if you're in 3D mode, it will call the 3D fillet tool. In the meantime, knowing the difference will save you a bit of frustration.