Recently I've been looking to build on my knowledge and add 9ths and 6ths into my blues playing. The part I'm struggling with is understanding why the 9th isn't just called the 2nd. I've read stuff about how it's to do with building chord voicings and them being based on stacking thirds to make chords but does this also apply when discussing scales? For example, if I'm playing the A minor pentatonic scale and I play the root note on the 10th fret and the 2nd string, then I'm playing the B on the 2nd string 12th fret, in what way is this different, if at all, from just calling it the 2nd rather than the 9th? I've read there can be a difference in sound between a 9th and a 2nd but how can that be possible here as it's not as if you're playing it up an octave?