On the Wikipedia page for Minor scale is this diagram of the harmonic minor scale starting at A:
Why is there a natural sign on the F? I thought the natural sign was only used when the note had been previously sharpened or flattened either via an accidental or as part of the key signature.
Just to be cautious in case someone is misled by the sharp on the G to think the F should be sharp, and also to be sure it is not confused with the melodic minor, which has an F# on the way up.
There is probably a natural sign given to the F to further emphasize that this is a harmonic minor scale instead of a melodic minor scale. The image from Matt's question is from a wiki page. In common musical notation, an F normally won't be labelled in the key of A minor.
If that F natural is so important to not be confused with F#, which would appear in melodic minor, why hasn't it been notated into the top line, second and fourth chords?
It's not necessary, but someone's being helpful - just like in a lot of music, where an accidental is put in a bar, then it's cancelled, i.e. with a natural in the next bar - the barline doing that automatically, anyway.
The letter name underneath is a good clue, too...
I would say that this distinction is made to separate it from the Melodic Minor scale. In the other chords with an F, it being natural is spelled out in the chord symbol (B dim would have an F by definition, same with the D minor). For the F chord, the chord symbol would not necessarily provide the distinction. I think that it was entirely unnecessary to add the natural to the chart unless it was being directly compared to the melodic minor scale but I'm sure at least one person found the distinction brought clarity to their understanding. When I make this type of distinction I refer to the chord as bVI.
