A student asked me how you know what key of piece of music is in. The answer is: by the Key Signature.
The Key Signature is at the beginning of the piece of music on the staff where there will be between zero and five **sharps (#) or flats (b)**written next to the staff symbol.
# means raise the note by a half-step b means lower the note by a half-step
The circle of fifths is a great tool for seeing and understanding key signatures. If you don’t have a circle of fifths, there’s a trick to telling the key by a the key signature.
The trick is:
For sharps: whatever note the last sharp is written on, the key is a half-step above that note. So just F is sharped = Key of G.
F and C are sharped = Key of D.
F,C, and G are sharped, Key of A.
For flats: if there is one flat it is the key of F, and for the rest of them the second to last flat symbol is on the note that the key signature represents.
So flats on B and E = Key of Bb.
Flats on B, E, and A = Key of Eb
Thoughts comments on key signatures and/or the circle of fifths?
Oh, yes, @Luke. I feel like I did before the math lesson and I’m looking for reasons to “skip”. In the end it’s no use to you and you have to go through… Without theory worse practice, I did it, with the help of my keyboard it went well. Greetings from Astrid
Hi @Luke. I have a framed circle of fifths on the wall in my music room. It’s the most useful music tool I have by far. Once you learn the basics of how to use the CoF, it becomes indispensable. Finding key signatures is just one of the many uses of the CoF.
Don’t know why musicians have ‘circle of fifths’ charts when it is so easy to remember with the following sentence: Funny Carol Gets Drunk And Eats Grasshoppers.
For flats: if there is one flat it is the key of F, and for the rest of them the second to last flat symbol is on the note that the key signature represents.
So flats on B and E = Key of Bb.
Flats on B, E, and A = Key of Eb
If that is still not clear then let us know what it is that you don’t understand and someone will try again.
Oh ya gotta love this mnemonic! Funny Carole Gets Drunk And Eats Grasshoppers. But hey! That’s 6 notes not 5 -maybe someone can provide an easy explanation? My guess is if you move up five or down five from a signature note ya need a 6th note.
In the end it’s no use to you and you have to go through… Without theory worse practice, I did it, with the help of my keyboard it went well. 
Greetings from Astrid 
