Identical by descent/13/en
From ISOGG Wiki
The segment boundaries are somewhat fuzzy. 23andMe round the segment length to the nearest tenth of a centiMorgan and round the segment start and end co-ordinates to the closest millionth base pair to reflect the uncertainty in the exact locations of the segment boundaries. Family Tree DNA round the start and stop positions to the nearest complete block of 100 SNPs on each end of the matching segment.
The origin of IBD segments is depicted via a pedigree of 12 individuals. Each box (male) and circle (female) represents an individual with two homologous chromosomes as bars. The top row shows three couples with their chromosomes colored differently. Due to crossing over, offspring inherit recombinant chromosomes of their parents. The first order cousins in the bottom row share one IBD segment (borders marked by grey lines). Both have inherited this IBD segment from the same individual, namely their grandfather (orange colored chromosome in the top row). Diagram courtesy of Gklambauer, Wikimedia Commons.