Thanks guys.
Does this mean that if i have 10gb worth of music, i'll actually have 20gb as itunes makes a double of every song?
Is it worth deleting every file in my music folder in hope to gain plenty of space on the computer and still have all my songs on itunes play?
Your iTunes library is effectively a link to all the music in the music folder on your mac (standard file path should be
Macintosh HD/Users/'User name'/Music/iTunes/iTunes Music). If you have music in another folder (e.g. Downloads) and add that music to iTunes, iTunes will then make a copy and categorize it into the iTunes Music folder. Therefore, now you have 1 copy of the song in the Downloads folder and 1 in the iTunes Music folder (which iTunes is linking to).
If you delete the one in the iTunes Music folder, the iTunes library file (list of all songs that comes up in iTunes) will be trying to link back to a file/song that no longer exists, it won't play in iTunes and iTunes will show an "!" next to the song because it can't find/link to it.
If you look at the size of your iTunes Library file, you may be surprised at how small it is. Mine is 1MB. This is because it is just a bunch of text and code to link to the actual song file in the iTunes Music folder.
Unless you have doubles of the music file elsewhere in the Finder, you would be completely deleting your music if you deleted it from the iTunes Music folder.
Get to the point... Hope that makes sense. Basically, don't delete from the iTunes Music folder in the Finder, because
that is the music in iTunes, it's not double counting.
However,
if the music you have in iTunes is in another folder in the Finder apart from the "iTunes Music folder" (Macintosh HD/Users/'User name'/Music/iTunes/iTunes Music),
then that separate copy is taking up unnecessary space.