Before the Shattering the Seelie and Unseelie Courts were constant rivals. Even with the agreements between the two courts that divided the year into two parts; Summer and Winter halves, conflicts were regular occurrences. The Shattering changed all of that. For survival's sake, the two Courts had to put aside their issues. The Compact was an unprecedented arrangement and the truce called for a ceasing of hostilities "for the duration," whatever that may mean. Seelie lands allowed Unseelie travelers. Unseelie lands played host to the Seelie. As the two groups mingled more and more freely, they adopted some of each others' customs and changeling society became a mix of the two ways of being. Seelie law, order, and formality learned to coexist with Unseelie disorder, chaos, and impulsiveness. This dynamic blend of opposites has persisted to the present.
See also Changeling History#The Compact