The 23andMe V3 chip was 23andMe's third microarray chip. It replaced their V2 chip in January 2011. It was a custom version of the Illumina OmniExpress Genotyping BeadChip. One of its key features was backwards compatibility with their V2 chip.
Microarray chips test a small percentage of SNPs on the human genome. Each letter the DNA code for the human genome (A, C, T, or G) is called a base pair. This one tested about 960,600 SNPs.
SNPs per Million Base Pairs – Heatmap
The V2 chip tested 960,600 base pairs across the human genome. This heatmap shows how those SNPs are distributed across the 22 autosomal chromosomes and the X-chromosome. The ranges are for counts of tested SNPs in blocks of 1 million base pairs.