C9

C9 is a designation used in several distinct domains, so its meaning depends on context. It can refer to a component of the immune system, a musical chord, a type of decorative lighting, or a chemical descriptor for hydrocarbons containing nine carbon atoms.

In biology and medicine, C9 refers to complement component 9, a serum protein that forms part of the membrane attack complex in the terminal complement pathway. Working with other complement proteins (C5b, C6, C7, and C8), C9 polymerizes to create pores in the membranes of targeted cells, contributing to immune defense. C9 deficiency is rare but associated with increased susceptibility to invasive Neisseria infections.

In music notation, C9 denotes a C dominant ninth chord. This chord combines the triad C–E–G with a dominant seventh (Bb) and a ninth (D). The basic voicing is often C–E–G–Bb–D, though players may omit certain notes or alter tensions depending on style and arrangement.

In consumer lighting, C9 refers to a class of large Christmas light bulbs used for outdoor decoration. C9 bulbs are known for their bright appearance and are commonly strung on outdoor displays and holiday lighting strands, compatible with standard mains voltage in many regions.

In organic chemistry and petrochemicals, C9 indicates molecules containing nine carbon atoms. It is used to describe nine-carbon hydrocarbon fractions, including nonane isomers, and to categorize certain chemical species or fractions in refining and synthesis discussions.