A frontend connects to a running client (for example via JSON-RPC) to provide a user interface for it, it does not implement the bitcoin protocol itself.

See Also

API Bindings
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Bitcoin Core
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Bitcoin Explorer
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Bitcoin XT
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Bitcoin-JSON-RPC-Client
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Bitcoin-js-remote
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BitcoinApp
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Bitcoiner
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FastCat BRPC
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Java Bitcoin Client
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Obelisk
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Python Bitcoin
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Python Bitcoinclient
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SubvertX
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Wallet.Net
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