The Rule of 9 in American Sign Language (ASL) is a term that describes a rule or pattern in numeral incorporation that a number only up to 9 is incorporated with a regular sign, usually related to time with a few exceptions.
For example, the ASL signs two and day are two separate individual morphemes, meaning "two days".
In numeral incorporation, the number two is incorporated with the sign DAY. It becomes a single sign, but it has two meaningful parts (morphemes). The numeral part is a "bound morpheme."
This can be done with a number between one and nine, but not beyond 10. That's the Rule of 9. For 10 days or above, sign two separate ASL words. E.g. TEN + DAY.
Another example, for the phrase "three weeks", the ASL sign WEEK is integrated with ASL number THREE. It can work with only numbers up to 9.
This Rule of Nine can apply to age, o'clock, minutes, hours, days, weeks, and months. Numeral incorporation doesn't occur in YEAR.
When writing glosses, the dash between two English words means one ASL word. The gloss TWO-DAY means one sign or word in ASL. Whereas, the gloss TWO+DAY with the 'plus' symbol indicates two separate signs.
Related topic: numeral incorporation