If I match Cousin A and Cousin B on chromosome 6, does that mean that we share a common ancestor? The answer is a bit complicated! First, the general answer: General rule: If you and cousin A and cousin B all match across a significantly overlapping span of the same copy of the same chromosome (called a segment), it is highly probable that you do so because the three of you all inherited that segment from a common ancestor. But what does that mean? First qualification : It's not enough that the matches both match you on the same chromosome. The matches must also match you over the same span of that chromosome. Position on a chromosome is generally measured in MBP or Mega-Base-Pairs. If Cousin A matches you on chromosome 6, but over the span of 23 MBP to 51 MBP, but Cousin B matches you over the span of 63 MBP to 81 MBP, this is not evidence that you and both matches share a common ancestor because these are segments to do not overlap....