A major scale flat and natural accidental

Question

Wikipedia states the following for the A major scale

The key of A major is the only key where a Neapolitan sixth chord on 2 requires both a flat and a natural accidental.

Can anyone explain what the above means and how does that relates to the scale on a guitar

Answer 1

The Neapolitan sixth chord is a major triad with its root on the ♭2 of the scale, most commonly played in first inversion. So in A major it is a Bb major triad, which in first inversion is:

D F Bb

Since the A major scale contains an F# and a B, in order to get the required F and Bb you need one natural (for the F) and one flat (for the Bb).

Answer 2

A Neapolitan sixth chord in A-major (3#) needs a natural (F) and a flat (B) accidental:

Bb  
F♮  
D    

D  

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Cadence:  

A  Bb_G# A  
E  F♮_E  E  
C# D  D  C#  

A  D  E  A