Very simple
1) Choose a key -That is just a note, like 'a'
2) Choose a scale -That is a set of notes that goes great together, and very easy done in LMMS
So lets set the scale aMinor also known as Am
Open Piano-roll
From the scale-dropdown select "Minor"
Point at semitone a in the piano-roll keyboard
right-click
Select Mark-this-scale
The result is that all notes that belongs to the aM scale are highlighted!
when you compose in Am then you ONLY use highlighted notes
As easy as pie
Want more?
https://www.youtube.com/watch?v=WhkwmAB ... cH1Rs56V8c