This article explores, from the point of view of both law and linguistics, how far the application and effect of the law of registered trade marks is shaped not only by legislative initiative but also by changing consumer behaviour and the shifting linguistic currency of the particular signs used (or proposed for use) as marks. It does so by focusing on the thirty-year campaign to register HAVE A BREAK for a chocolate bar, marketed as ‘KitKat’. It considers the changing approach of courts both to inherent distinctiveness and to distinctiveness acquired through use. It also considers the relationship between the average consumer test for distinctiveness and the public interest in leaving certain signs free. It suggests that while the present trade mark regime is open to the registration of slogans, it is not clear that courts have sufficiently considered the public interest implications of increasing trade mark protection in this way.