agree with ls BUT: if you change involves an intra-London train, then you may need to swipe you card in between. for example, if you travel from King's Cross to Greenwich, and you take a tube from King's cross to London Bridge, and take an intra-London train from London Bridge to Greenwich, then you need to swipe in and out in London Bridge as well. Usually, using intra-London train can be more expensive than tubes.
BUT: if you change involves an intra-London train, then you may need to swipe you card in between. for example, if you travel from King's Cross to Greenwich, and you take a tube from King's cross to London Bridge, and take an intra-London train from London Bridge to Greenwich, then you need to swipe in and out in London Bridge as well. Usually, using intra-London train can be more expensive than tubes.