Been thinking too much again!!!  🙄

I don't have any issues with Key Signatures for Major/Minor scales & Modes. 

BUT, what about tunes that don't have the usual 7-note scales? 

I've run across trad tunes that are made of the Pentatonic scales.  They never have the same amount of accidentals as their 7-note scale counterparts, e.g., D Major:  

• D Major has 2 sharps (F# & C#) - sounds very happy 
• D Major Pentatonic has only 1 sharp (F#) - sounds almost more happy to me, even with the C# missing. 
• D Mixolydian has 1 sharp (F#) - would be considered the mode for a tonal center of D with 1 sharp... it's the neighboring key to D Major, but there's a definite feeling change, it's just not as close to the Pentatonic. 

So, I understand Key Signatures tell us what accidentals are in use - but I look for it to help me narrow down a scale & which chords may be used.  When I figure out the Key, I can consider neighboring keys to borrow chords & notes, too. 

I'm just wondering if trad folk musicians would just use the Key Signature the Pentatonic was derived from & ignore there's any missing notes/accidentals (C#  D Major) - OR, could it be more helpful to show the missing accidental as a 'natural', e.g., F# and C having a natural sign? 

I'd like to know what people think here (on the forum), because my query could be directed at BLUES SCALES, too - since they only have 6 notes. 

I saw this video today, so I asked about it in the comments... waiting for a response. 

The Celtic Pentatonic Scale - tutorial from Clíodhna Ní Aodáin.