Last week, I discussed one of the complications of doing research in French Canada (here). In addition to the widespread variations in spelling, French Canadian surnames present another challenge for genealogists. One common practice in North America was to use dit-names in addition to the surname. The word "dit" means "said" and can be translated as "called". For example, one of my wife's lines went by the name Boutron dit Major, Boutron being the surname and Major being the dit-name.
A common explanation for this practice is that the names were adopted in order to distinguish families carrying the same name. However, you will find that this practice is very widespread covering both common names and rarer ones, so it is probably best just to think of this more as a fashion that became popular in this part of the world. Despite how common this is in Canada, I have yet to encounter it in my non-Canadian French ancestors.
In many records you will find the full name given, the surname with the dit-name. In these cases, the dit-names can be useful, helping you identify the correct records. What makes these types of names more difficult is that either the dit-name or the surname is often omitted. In some records you may find members of the Boutron dit Major family listed simply as Boutron or simply as Major. When French Canadians migrated to the United States, sometimes they kept the surname and sometimes the dit-name. After they moved to New York, my wife's ancestors used "Major", a name which needed no anglicization.
There are two other complications with dit-names. First, for women, "dit" takes the feminine form "dite", Boutron dite Major. Second, the dit is often omitted giving you Boutron Major. Make sure you check all the possibilities.
If you are working back from the US to Canada, it is helpful to determine whether the surname derives from the original surname or a dit-name. A helpful online resource is provided by the American-French Genealogical Society. If you look up a name, you will find a number of associated names, this includes spelling variations, common anglicizations, dit-names associated with particular surnames and surnames associated with particular dit-names. Unfortunately, the database does not indicate the relationship only that the names are associated. The resource is also helpful in cases where a name is difficult to read in a record. If the surname is illegible, a legible dit-name may be the clue you need to connect that person to someone in another record.