# What is the difference between a single and dual indexed library?
**A single-indexed library** has only 1 index, specifically in the p7-side adapter. This is called the **i7 index** and is sequenced during index read 1 of the sequencing run. A single-indexed library must have the index in the p7 adapter to sequence on Illumina instruments.
**A dual-indexed library** has 2 indexes, index1 (i7) in the p7-side adapter and index2 (i5) in the p5-side adapter. Dual-indexed libraries can either be combinatorial dual (CD) or unique dual (UD). This concept is explained in [Understanding unique dual indexes (UDI) and associated library prep kits](https://support.illumina.com/bulletins/2018/08/understanding-unique-dual-indexes--udi--and-associated-library-p.html)
**Note**: On Illumina instruments, it is not possible to sequence an i5 index without first sequencing the i7 index (ie, a single-indexed run only sequencing the i5 index is not possible).
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