From The Moderate Voice:
Modern music can make it in the Great Music portfolio because modern musical instruments not only sound better than the old ones musicians had to play in the distant past but also because modern musician were able to use different rhythms, chords and instruments.
For example during the Classical period of music, there were thousands of rules about what could and could not be done in composing music. The Italian Court was in charge of establishing the rules and the way they controlled musicians was by prohibiting the publication of music that was against the rules. Illustrating this point was Beethoven’s Moonlight Sonata where in the first movement Beethoven wanted to play a natural G in a section of the music but that was against the rules because a natural G does not appear in a C# minor scale, the key signature of the piece. Beethoven wanted the music to be published but also wanted the music to be true to his intentions and use a natural G.
So how did Beethoven get around the rules?