physical buttons, or lack thereof

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http://androidforums.com – I'm new to Android, about to buy a Captivate Glide. I had a question about physical buttons... Some phones have four buttons, others three... and some of them none. How do the phones with no buttons handle legacy apps that are programmed to use the hardware buttons? Like if an old app is programmed to use a hard menu button, what do you do on a newer phone with no buttons? Conversely, new apps that are not programmed to use hard buttons... do the hardware buttons become useless on older phones? Or are apps programmed to recognize whether or not there are hardware buttons or not? T (General)