1.  You share a common ancestor before the establishment of surnames

2.  Convergence:  where both participant's result mutated and now match

3.  An adoption in one or other of the lines

4.  An extramarital event in one of the lines

5.  A branch of the family adopted a different surname

Matches with other surnames are typically more prevalent with those who are Haplogroup R1b.

Most likely, when you match some one with a different surname, you share a common ancestor before the establishment of surnames or convergence occurred.

Imagine a situation 3 generations before surnames were established, where our imaginary ancestor "Robert" had 5 sons from his first wife, who then died, and 2 sons from his second wife.  In the next generation, these 7 sons had a total of 27 sons who lived to adulthood.  These 27 sons then had 108 sons.

Each of these 108 sons would have Robert's Y-DNA.  Over time, many of Robert's sons moved away to other villages, often when they married, and sometimes they were looking for a better situation.  Many of Robert's grandsons then moved away from their father, maybe only to the next village.  The 108 males in this family tree are now spread out geographically.  A few had even traveled a long way from their ancestral homeland looking for a better situation.

As surnames became established, it is very possible that most of Robert's grandsons ended up with different surnames.

Robert's descendents are not the only ones with the same Y-DNA.  Robert also had 3 brothers, who had descendents.  Robert's father had 4 brothers, who also had descendents.  Robert's grandfather also had brothers who had descendents.  There were many males who had Robert's Y-DNA, or a close result if there had been a mutation.  Each of these males could have adopted a different surname.  At the minimum, those in different places most likely would have taken on a different surname.  Also, those who did not know that they were distantly related probably took on a different surname.

Today, there would be many males with Robert's Y-DNA - and a wide variety of surnames, spread over an even larger geographical area.  Since these matches are before the adoption of surnames, they are most likely not worth pursing for your family tree.  The value of these matches is that they could provide clues regarding the ancestral location, for those that have not been able to discover the ancestral location.

Another event that can result in Y-DNA results matching for males with a different surname is called convergence.  Convergence is a scientific term that applies when two Y-DNA results have mutated so that they now match each other.  See  Haplotype Convergence for a further explanation for this situation.

Adoptions occurred in the past, although they weren't necessarily as formal as procedures for adoption are today.  A widow could remarry, and the children took on the surname of the new husband.  A child could be abandoned, and a family took in the child, and the child assumed the surname of the new family.  Before pursuing Y-DNA matches with another surname based on assuming that an adoption occurred, first review your family history research to determine if there is any evidence to support a possible adoption.  For example, do you have a widow remarrying and the new husbands surname matches one of those surnames of the DNA results that you match?  Do you have a child in your direct male line who appears in a census, yet you can't find the birth record?  Are any of the surnames with your DNA result matches found in the locations where your ancestors lived? Have any children disappeared between censuses, and you do not find a death record?

If you don't have any evidence of an adoption in your family tree, then it probably isn't worth pursuing a Y-DNA match with another surname under the assumption that there is an adoption.

Extramarital events occur, including illegitimate births.  Extramarital events where the female is married will be the most difficult to track down.  For an illegitimate birth, typically the Parish Registers will note that the person being baptized is illegitimate, and only rarely does a Parish Register indicate the father.  Often, even the death of the person will indicate that they are illegitimate, since illegitimacy carried such a stigma for the person's whole life.

From your family history research, you would most likely know if your direct male line includes an illegitimate birth.  You have probably also validated the Y-DNA result for your family tree, so you would have identified a problem if the two results didn't match, and most likely have undertaken additional research and done additional testing to resolve the situation with your family tree.  If you wonder if an illegitimate birth occurred further back in your family tree, then your best course of action is to pursue research to take your family tree back further, before pursuing matches with other surnames.

Extramarital events where the female is married are much more difficult to track down.  There must be some evidence to make this conclusion.  For example, did the descendents of the first son match others with the family surname, and descendents of the last son don't match the surname result?

Was there a later divorce and remarriage?  If so, does the surname of the second husband match any of those surnames for the Y-DNA match?

Another event that can result in Y-DNA matching others with a different surname is when a branch of the family tree takes on a different surname.  There are many reasons why a surname could be changed.  Perhaps, it is simply personal preference, or the family immigrated to a new country and wanted to fit in.  A husband could take on the wife's surname, to prevent her surname from becoming extinct in her family tree.  The surname could also have evolved into a different form when migration is combined with illiteracy.  The person migrating could only say their surname, and the spelling could be dramatically different in a new location with a different language or accent.

Most likely you would have some clues in your family history research as to whether a different surname is possible.  Do you have a missing person of family group?  If all the people are accounted for, then most likely, assuming a different surname by a branch of your tree is not the reason that you have a Y-DNA match with a different surname.

Most likely, Y-DNA matches with other surnames are a result of being related through a common ancestor prior to surnames, or through convergence.

A match with another person is always exciting.  The question then becomes, do you pursue the match?  The first step before pursing a match is to upgrade your test to 37 Markers, to see if the match still occurs.  In most cases, there will no longer be a match.  The next step would be to review your family history research to determine if there is possibly an adoption, surname change, or extramarital event.  If you don't find any clues to support the possibility of these events, then it is reasonable to assume that the Y-DNA match came from a common ancestor prior to surnames or convergence.

Those who are Haplogroup R1b will tend to have DNA matches with other surnames.  Haplogroup R1b is the largest population group in Europe, therefore, due to the size and scope of this large population, there have been many opportunities for convergence.

If you belong to a Surname Project, you can eliminate seeing Y-DNA Matches with other surnames by setting your Public/Private setting (on your Personal Page at FtDNA.com) to Private.

The Public/Private setting determines whether the search for Y-DNA matches will only look for matches within the particular Surname Project or if it will look for matches in the entire Family Tree DNA customer database for those participants with a Public setting.

The Public/Private setting is established for each individual participant.  A Surname Project Group Administrator may encounter different DNA Lines or Groups within their project.  Perhaps one Group has Y-DNA matches with other surnames.  This DNA Group could be set to Private, and all the other members of the Surname Project remain as Public.

If you have questions concerning your need or desire to have a Public or a Private setting, contact Robert B. Noles, the Group Administrator for Knowles Surname Project.