#1
Ive heard that there is 2 melodic minor scales... one ascending and one descending...

question is: if you play the scale ascending, if you wanna go back, do you have to do the descending version or can you just stick with the ascending version?
#2
In classical theory: The ascending version is played when ascending, the descending version is played when descending.

In jazz: The ascending version.

Would you like an explanation of the why of the melodic minor scale?
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#4
Well, it all starts with the harmonized minor scale.

Out of that was born the Harmonic Minor scale (by changing the V-7 to a V7 chord).

The Harmonic Minor scale has a big interval between the 6th and 7th degrees. In
order to improve the "melodious" nature of the scale and reduce that gap, the
6th was raised a 1/2 step. That's the Melodic Minor scale. That still retains the V7
when you harmonize the Melodic Minor scale.

The 4th mode of the Melodic Minor scale is a popular jazz scale substitution over
Dom7 chords. It's called the Lydian Dominant scale (nature of Lydian = #4, nature
of Dominant = b7). The 5th mode, Mixolydian b6, could also be used.

Another way to look at the Melodic minor is that it's the same as the Major scale
except for a b3.