On guitar you have to learn each key SEVEN times, though, through all the different positions.

But my point is this: it SEEMS like a lot of work, but it really isn't if you understand music! If you can sing Do Re Mi Fa So La Ti Do and know that every step up is a whole step except between Mi & Fa, and Ti & Do, it's trivially easy to build the scale on the fly, and if you screw it up, your ear will guide you back on course. And I do use note names as a guide posts.

On any instrument, you can play completely by ear (eventually), but a little knowledge goes a long way.

On guitar, you can play a whole major scale octave with the pattern 0-2-4-5-7-9-11-12--which you can figure out yourself either by ear or by applying the major scale formula. That immediately gives you access to an octave in A, D, G, B, and two octaves in E (open strings). Then you can start playing around with a low E drone and a high E melody, and the fifth, B, is happily always one string below you. Then maybe you notice the A, the fourth, is the next string below the low E and you can hit that every once in a while, maybe while playing the notes of the A major triad, A C# E; you don't need to memorize them because you know the fourth starts on the fourth note of the scale, which is the 5 fret, and then you skip every other note to get the triad--5-9-12. And for a little flavor, you can throw in a major seventh, which you know is one fret below the root, 4. And you would also know that the octave starts at again as 12 so you can also use the 16 fret (12+4).

If you keep yours wits about you, maybe while you're noodling, you try to keep note name in mind, so pretty soon you know where E B and G# are on the high E string, and merrily you go along, experimenting, and playing, and learning, and having a good time.

Contrast that with the traditional learning method of: ¨THIS musical note corresponds to THIS string at THIS fret position. Keep drilling until it sinks into that thick skull of yours. I will be judging you on your performance of this scale next week.

No wonder people think learning a new instrument is an incredible chore!

Edit: My FAVORITE isomorphic keyboard are the button accordions: French and Bayan (they go in opposite directions). Basic version is a a stack of three buttons, each one a semi-tone higher than the last. So you really only have to memorize three scales, for starting on each of the three different buttons. Fancier version add duplicate rows for ease of fingering.

That said, it's much less of a hurdle if you have a good ear and can immediately correct yourself and say, "ooops, that ain't right."