What Is Blood Sugar And Why Is It...
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No best answer has yet been selected by dave_c. Once a best answer has been selected, it will be shown here.
For more on marking an answer as the "Best Answer", please visit our FAQ.Basically in 2 ways,
You either buy a monthly pass - like the old travelcard system except the info is logged electronically on the Oyster card, by the card reader (yellow circle thingymajig). Then say you have a zone 1-6 pass, you use it as normal for one week, month, year etc.
Pre pay/top up - you 'charge' up your Oyster card with say �20 and you can travel as much as you like until the �20 runs out (the card is read by the card reader at every tube station and on every bus etc). They have introduced a capping system now, so that the daily amount doesn't exceed the maximum you should pay for a day using any of the London transport network (tube, busses, trams, DLR and national rail networks as applicable).
More stuff & nonsense here: