by the tradition and custom of the world, a woman is supposed to take the name (surname) of her husband in marriage. this is to show that she is no longer single and she is for her husband. just as a man sacrifices his friends and family (which the woman also does), it is a world wide practice for a woman to take the name of her husband.
the law however does not make it mandatory. it is by choice. that is why i woman who was already a public figure with her name may not change her name after marriage because of public policy. imagine your icon or supper star having to change names, it may not be easily accepted by the public - it won't be easy for them to get use to it and it may pose some difficulty for your company to go through all the change of name processes
legally, it is not mandatory, conventionally, yes!
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