Ruy Lopez Main Line 
With White preparing to strike in the centre with d4, Blacks king typically vacates the premises with 8...O-O. The old-fashioned move is 8...Na5, which may transpose, but the loss of control over d4 may allow White to save a tempo by omitting h3.
8...Bg4 is seen from time to time, discouraging an instant d4 by White. White will probably play d3 as a temporary measure, aiming to bring the queens knight to the kingside to evict the bishop in order to achieve d4.
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- Modern Chess Openings 15th ed, 2008. Nick de Firmian. Random House, New York. ISBN 0-8129-3682-5.