Say, you've played guitar for a few years and you want to play another instrument like piano, bass or harmonica, is it easier to learn them, in particular bass?

To be honest I don't think I'll settle with being able to play only guitar.
yes it'll be significantly easier, if you're learning a lot of your musical theory and note reading as well
Bass will be a little easier because you already have some strength in your fretting hand fingers.
Yes, alot easier. Since you've been playing guitar, your rhythm skills have probably improved, making you better at other instruments timings. Drums will be easier to keep steady. I don't mean you'll be a natural, but rhythm will be great.

I play drums, saxophone, guitar, and piano.
i play a trombone, piano and euphonium it helps bass i think would be the easiest for you to jump into bass as you know note reading(tabs). But honestly to enhance your skills in most instruments piano all the way .
Fender Telecaster
Squire Vintage Modified '70s bass
well as far as bass, you'll have 2 less strings to deal with and should already to know the notes that go along with it. however, it can be refreshing to try something totally different. i've messed with keyboards, drums, mandolin, and violin to dif levels of success. i made more noise than anything, but had fun trying. good luck
All my guitars are old enough to buy beer, are You?
It's usually harder - reason being that your prior knowledge often leads you to expect it to be easier than it actually is. Prior theory knowledge helps a little in terms of getting to grips with things on a purely musical level, but as far as technically is concerned being able to play the guitar already makes no difference whatsoever....you'll be a stone cold beginner at the start and it'll be just as if you'd neve picked up a musical instrument before in your life.

The only difference is bass, because the bass is, to all intents and purposes, a guitar. Everything's laid out the same and the basic mechanics are the same, however you have to be a fair bit stronger and also you tend to need to be a far more considerate musician to play bass effectively.
Actually called Mark!

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