Changing layout on multiple devices?

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http://stackoverflow.com – I'm sure there's a basic answer but I'm still learning and I'm not quite sure. I have an app with a few button on the start screen. It works on a Nexus One (emulator) and a Samsung S2 (device); probably because the screen size is the same. However, when I preview all screens on tablets for example the button are completely spread out and really small. On other devices they're halfway into each other and just doesn't look good. I've tried match_parent and wrap_content but that doesn't help, how do I use some .xml code or layout properties to make sure the button's don't budge into each other ye (HowTos)