To demonstrate this, we will use the key of C Minor, which has three flats.
Instead of writing a flat next to every E, A, and B; we can simply add a key signature to the beginning of the measure.
Next, let's try A Major, which has three sharps.
Again, a key signature can be used instead of writing a sharp next to every C, F, and G.
As you may have noticed, the sharps are arranged in a special order.
F# comes first, followed by C# then G#.
Next comes D#, A#, E#, and finally B#.
You can remember this order by using the following saying: 'Father Charles Goes Down And Ends Battle'
Starting with Bb, the order of the flats is the exact opposite.
Next comes Eb, Ab, Db, Gb, Cb, and finally Fb.
This order can be remembered with another saying: 'Battle Ends And Down Goes Charles' Father'
If you want to test your knowledge of key signatures, try our Key Signature Trainer