When you buy a single park ticket it is good for either park.
There are different kind if tickets. The first option is the single park per day tickets. These will allow you to buy tickets for as many days as you want and chose 1 park per day to enter. So if you have a 3 day ticket, you could go to Disneyland the first day, Disney California Adventure the 2nd day and they choose to go back to either park the 3rd day. A single day ticket lets you choose the park to enter. The other option is to get a Park Hopper ticket. You can get these for as many days as you want. With these you can go to both parks in the same day and jump back and forth as much as you want. It will even allow you to get a Fastpass for a ride in one park and wait to use it while you are in the other park. It basically turns both parks into one huge park. I always prefer this choice because if one park is super crowded, the other may be slower. If you download this Disneyland app you can see the ride wait times, find dining options (with menus and prices) and use the map.
For California adventure the cost 90 and up
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