The Ancestry V1 chip was Ancestry.com's first microarray chip. It was a standard version of the Illumina OmniExpress chip Genotyping BeadChip.
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 701,400 SNPs.
SNPs per Million Base Pairs – Heatmap
The V1 chip tested 701,400 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.