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The number9(九jiŭ), was historically associated with the Emperor of China, and the number was frequently used in matters relating to the Emperor, before the establishment of the imperial examinations officials were organized in the nine-rank system, the nine bestowments were rewards the Emperor made for officials of extraordinary capacity and loyalty, while the nine familial exterminations was one of the harshest punishments the Emperor sentenced; the Emperor's robes often had nine dragons, and Chinese mythology held that the dragon has nine children. It also symbolizes harmony.
Moreover, the number 9 is a homophone of the word for "long lasting" (久), and as such is often used in weddings. It is also a homophone for the words "to have enough", "to save". Hence, when you put it with other words that are lucky, it emphasizes the benefit of that number 89 (To have enough luck), 29 (to easily have enough), 39 (grow enough). Here the word enough means more "to have that which you need to achieve your goals" vs. "just enough." When combined with the number 4, it's still not necessarily auspicious.