Amateur or NN is only used if the player's name is unknown. Chances are they'll lose against an established master anyway but, if they do win against a very strong player, their name does tend to get remembered and will be used on the score.
Does Amateur ever win?
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A lot of games are lost by Amateur or NN.
This is because losing, even an amateur to a known master, is shameful - an amateur who loses is not named.
But if an amateur wins, is he/she still quoted as Amateur/NN, or is he then named because winning is a honour to be named?