I play piano, if i teach myself to play guitar will it mess me up on piano?

The reading part will be a little different because guitar is written one octive higher than it sounds. Also, when going to guitar from piano many notes can be played all over the fret board and they sound the same compared to piano the notes can only be played in one spot. But other than that just apply your theory to guitar and have fun!

I am a beginner with electric bass. I originally set out to year keyboard, but after about a year I switched to electric bass. It is very helpful to have thin fingers for keyboard. This is because when playing chords your fingers often have to fit between the black keys while stretching to reach other keys required for a chord. Keyboard (and piano) is something that art musicians (i.e. classical) love. For that reason a lot of the instructional materials and instructors are centered around art music and drippy folk music. It is much harder to get through a lesson if you don't like the song your learning. If you just want to learn blues or rock, they will tell you to learn all this other stuff "because it will make you a better musician". The positive for keyboard is that you can play so much with it. You can lead. You can accompany. You can solo. it has a huge range. If you get an electric keyboard you also get a huge selection of timbres and a mind boggling collection of other features. Especially with an electric keyboard you can easily give the rock guitarist a run for his money. Bass is much easier on those of us with thick fingers. (pay attention to the neck size when purchasing. The shorter the neck the closer the frets. However a lot of people like the shorter necks cause then you don't have to reach as far.) The other thing about bass is much of the instructional material is rock or blues. There are many books that won't even require you to read music. They use a system called tabulature or tab. The negative for bass is that there is a lot less you can do solo. Also, you have to strap it on to you, and it can be very heavy and painful.

I've always heard it makes you a better player to learn piano first.

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no i play both and it doesnt make much of a difference

No. It will actually make you better.

No