Some times when there are so many sharps and flats in a piece that writing them as it is will look too cluttered or clumsy, these sharp and flat symbols (# and b) are placed before the bar itself, next to the Clef and the Time signature. The lines or spaces, on which these are on, now indicate sharp or flat notes, as per the sign before it. This allows a composer to write a sharp or flat note on the staff by using just the time notation and not having to go on writing the sharp or flat sign before each note.

Here, there is another problem, that being how one may denote a natural note. This is where the natural symbol ()is used. Like the sharps and flats, it is placed before the note. Key signatures simplify the writing many folds.

For example, the key signature used for a song written in C# and Cb (B) are as follows:

An example of a key signature usage is as follows. The one above has no key signatures but the one below does.


Example with a Simple Song: Twinkle Twinkle

http://basicmusictheory.blogspot.com/2008/12/key-signatures_3731.html