In Diablo 3 for the PC, by default you can’t select two skills from the same skill group. Blizzard said it was so that new players wouldn’t make big blunders and choose a bunch of skills that don’t go together well.
That’s where Elective Mode comes in. If you go into Gameplay Options in the menu, you’ll see Elective Mode as a checkbox with very little description. Elective Mode allows you to select any skill from any group in the skills list.
You can click the below images to make them larger.
If you look at the below picture, you’ll see that no matter which skill you open up, now that you have Elective Mode turned on you’ll have access to left and right arrows that let you browse other skill categories. So for example, instead of having a secondary skill on your right click, you can put another primary skill there instead.
That’s all there is to it, now you’ll be able to setup skills like a champ. I’m surprised that Blizzard hasn’t made this extremely obvious, but I think they’re planning on making people more aware of it in the future.
