In a context of competition between exchanges, most European stock exchanges have now chosen order-driven systems for the trading of large capitalisations, yet some organisation specificities have remained, and the comparison of trading costs between order books with different trading rules has become extremely relevant. Therefore, the aim of this research consists in comparing spreads on two different types of auction market: the hybrid SETS (Stock Exchange Electronic Trading Service) electronic order book of the London Stock Exchange (LSE), for which UK Blue Chip stocks may be traded with dealers off the order book, and the almost pure centralised NSC (Nouveau Système de Cotation) electronic order book of Euronext, for which a concentration rule applies.