Complex sentence: The Upper Canadians valued their common law legal system, as opposed to the code civil of Quebec, which had chafed them ever since 1763, ostensibly one of the primary reasons for the partition of 1791.
Split into simpler sentences: The Upper Canadians valued their common law legal system, as opposed to the code civil of Quebec, which had chafed them ever since 1763. This was ostensibly one of the primary reasons for the partition of 1791.