Im completly new to theory and already it feels like my head is going to explode so please keep your answers simple. I'am looking at the notes which make up a key signature in a book. Does the key signature determine the key of the song and if I wanted to write a song could I only use the notes (chords) which are deterimined in a certain key signature to keep it in key?
It will tell you the notes in the key, but not the key itself.

E.g. A key signature has one sharp [F#]. It could be E minor or G major.

And you can use outside notes. They're just called accidentals.
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Keys signatures show the key by showing which notes are sharpened or flattened. This lowers the key selection to two options in more music, the Major or relative Minor. And no, you are not limited to these notes. If in minor, you will often find the 7th note to be sharpened (or natural if flattened in the key). This gives it a stronger pull to minor. So no, you are not limited at all
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Quote by hellybelly
Does the key signature determine the key of the song
Almost. It helps you to identify the key.
and if I wanted to write a song could I only use the notes (chords) which are deterimined in a certain key signature to keep it in key?
Yes, but you don't have to.

Simply put, just like the english alphabet uses exactly 26 letters out of thousands of characters and symbols available, a key determines 7 notes you can use out of twelve existing in western tonal music.

But not just any 7 notes. All the notes in each key follow stickt patterns. These patterns are the intervals. There is a recurring system that you can find in all the note sets of all the keys. That's what we call the scales.

To uphold the intervals in the different keys, some notes require to be flatted or sharpened. Now, you can write music in a certain key by adding the required flat or sharp symbols everywhere in your score. Or you can write them at the beginning of each bar, and leave them out in the score. The second option is what we call the signature. It's just more practical this way.

So, why is your first statement not completely correct? Well, there are at least two keys that have the same key signature. So in your score you have to look for other clues to find the one key that is used. The key signature in itself is only there to help you play the correct notes.

And what's up with the second statement? Sometimes you're allowed to use notes out of the key-set, while technically still being in the key. And if you want, you can change keys altogether in a piece of music. This is called modulation.

"Completely new to music theory" can mean you don't know how to read music. Or it could mean you can read notes, but don't have an understanding of their harmonic/key signature context.

If you could tell us about more about what you already know, that could help focus where you need to take your learning next. For example, if someone asked you to form a D major triad, would you know the notes that comprise it are D F# and A?

To answer one of your questions: YES, you can definitely create a song completely in-key, both in the chords and the melody.

Bob Dylan's Knockin' On Heaven's Door is a good example.
Last edited by joesix at Jun 12, 2011,
thanks guys this is really helping. im going to print this off. I cannot read music at all and completly new to theory, really know zilch.
Quote by hellybelly
thanks guys this is really helping. im going to print this off. I cannot read music at all and completly new to theory, really know zilch.

try this website, it can help tremendously with learning sheet music and theory. www.musictheory.net go through all the lessons you can bare to learn, but make sure you LEARN and UNDERSTAND them. I went through it once, just glazed through it, and it didnt help at all. after I actually paid attention, it became clear
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Quote by Woffelz
It will tell you the notes in the key, but not the key itself.

E.g. A key signature has one sharp [F#]. It could be E minor or G major.

And you can use outside notes. They're just called accidentals.
Mostly this, however, if a piece modulates (changes key) for a short time (especially if there is a series of modulations) then often the key signature won't be changed but the key will have, so the key signature will not always tell you the notes in the key (though usually it will).