Generally speaking the Mother's tartan was worn if she had married a non-Scot. So, if Isobel Ramsay married Guido Panzenetti, their son (little Archibald) would wear his Mother's tartan. This harkens back to the ancient Scottish custom of adopting "strangers" into one's family or clan. What you have to remember is that not only is little Archibald "entitled" to his Mother's tartan, he can also take her family name as well, if he so chooses.