# Why do NEBNext indexing primers have sequence between the p5 oligo and index?

In a previous post I asked Why do NEB adapters have non-complementary sequence?

Since then, I realized that there is some other sequence in the p5 indexing primer, as well as in the p7 indexing primer.

Here is a diagram of the NEBNext protocol. The parts that I am confused about are in step 5.

1. My first question is about the p5 index. Why is there a sequence 5'GATCTACAC 3' between the p5 flowcell oligo and the index?

1. My second question is about both indices. Why is there no gap between the p7 index and the flowcell annealing oligo?

For reference, I fetched these indexing primer sequences from NEB on page 18 and 19 of NEBNext Multiplex Oligos for Illumina instruction manual.