https://alexcabal.com/creating-the-perfect-gpg-keypair/
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Create a regular GPG keypair. By default GPG creates one signing subkey (your identity) and one encryption subkey (how you receive messages intended for you).
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Use GPG to add an additional signing subkey to your keypair. This new subkey is linked to the first signing key. Now we have three subkeys.
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This keypair is your master keypair. Store it in a protected place like your house or a safe-deposit box. Your master keypair is the one whose loss would be truly catastrophic.