Expression is considered to be at the pinnacle of qualities for any artist let alone a musician. Regardless of the genre, instrument or language, every musician tries to express a certain story, concept or thought process in the music he/she creates. For a guitarist, to be able to carve the right chord backing or improvise a solo befitting a song is very very important. Regardless of how many scales you know or how advanced of a guitar player you are, to be expressive is its own cup of tea. 

This article gives you insight as to how you can become a more expressive guitar player, though the idea applies to any instrument.

http://www.songstuff.com/guitar/article/express_yourself/

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