Year 9 (IX) was a common year starting on Tuesday (link will display the full calendar) of the Julian calendar. In the Roman Empire, it was known as the Year of the Consulship of Sabinus and Camerinus (or, less frequently, 762 Ab urbe condita). The denomination 9 for this year has been used since the late medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.

In mathematics, the repeating decimal 0.999… (sometimes written with more or fewer 9s before the final ellipsis, for example as 0.9…, or in a variety of other variants such as 0., 0.(9), or ) denotes a real number that can be shown to be the number one. In other words, the symbols "0.999…" and "1" represent the same number. Proofs of this equality have been formulated with varying degrees of mathematical rigor, taking into account preferred development of the real numbers, background assumptions, historical context, and target audience.

9 (nine ) is the natural number following 8 and preceding 10.

Unicode has subscripted and superscripted versions of a number of characters including a full set of arabic numerals. These characters allow any polynomial,  chemical and certain other equations to be represented in plain text without using any form of markup like HTML or TeX.