Which seven notes matters. At least on a guitar you only get 7 or 12. Some pipe organs have 15 notes which allows them to sound much better when you choose the right one (but some keys are horrible despite having 15 notes to choose from where as the standard 12 is an acceptable compromise for anything even if it worse where the 15 work). Violin gives you infinite choices, and I've seen keyboards that have 50+ (look up microtonal music). You can also remove the frets from a guitar - I've heard of one person (exactly one person) doing that.