The DNA Comparison feature, available to 23andMe+ Premium members, shows you the exact location of the DNA that you have in common with another 23andMe customer, and lets you take the next step in comparing your DNA with other relatives and profiles with whom you are sharing. Select any profile you manage within your account or any connected profile, and compare with up to 5 DNA relatives or connections.
The comparison results in this feature display shared segments of DNA on separate lines representing each chromosome pair, and label the shared segments as half IBD (Identical By Descent) or full IBD. Because you inherit one half of your DNA from your mother and the other half from your father, IBD segments typically occur on only a single chromosome and are therefore labeled half IBD. The length of IBD segments is measured in centiMorgans (cM). Half IBD refers to the amount of the genome in centiMorgans (cM) that contains an IBD segment on either chromosome. The percent DNA shared in DNA Relatives is based on this number.
Your shared segments vary based on the closeness of your relationship with the matches with whom you are comparing. Closer relatives will share thousands of cM and many segments in common; more distant relatives may share only one segment. For some of your connections, if you connected outside of DNA Relatives, you may not share any segments at all.
In the image below, darker segments of the same colour represent fully identical segments (i.e. that individual shares DNA with the focal profile on both copies of their chromosomes at this genomic location).