Graham, As with most societies through history, the Scottish clan system was based on a patriarchal model. Here is a quote from the clan MacKenzie website...

# The rules of the Scottish clan system (I quote): Excepting the "district", "Caledonia" and "Jacobite" tartans, no one should wear a tartan to which he is not by name or descent entitled. To do so is ill-mannered, invites scorn, and is contrary to the whole principle of the clan system. Nor does one "select" tartans from this or that "line" of ancestors. The vital question is, "To which Clan do I belong?"

* 1. You "belong" to the clan of which you bear the name or sept name.

2. You have no real right to wear your mother's tartan unless you have taken her name.

3. You cannot belong to several clans at once.

4. Adherents (cliathe) of non-clan names are, as followers, sometimes allowed to wear the tartan (usually a hunting sett if any) and to become members of a clan society.

That said, there are other opinions on the web.

"Clan is a Gaelic word meaning children, but in this context is more readily translated as 'family'.
Scottish clans are comprised of a core family or families derived from, or accepted as being derived from, a common ancestor...Allegiance was generally given to a father's clan, but Celtic tradition includes a strong element of descent through and loyalty to a mother's line. The chief of a clan could also 'ingather' a stranger, of whatever family, possessing required skills. If this person maintained their allegiance and it was required, they would adopt the clan surname."

This all makes sense in terms of societies historically dominated by men. The opposite (right or wrong) has been a rare exception. But let's not lose our sense of perspective in the matter. As has been posted here and many other places, unless you are a strict traditionalist, you wear the tartan that has the most meaning to you or that you find the most attractive. The meaning of the term "entitlement" with respect to tartans obviously depends on how much of a pragmatist you are.

Blu