Is it possible to have more than one private/public key pair per user for SSH?

I need a user with more than one key-pair for SSH. Is that possible? Is that possible without having to specify which key to use on each command?

From serverfault J. Pablo Fernández

• You can have multiple SSH keys (multiple identities). To use the at the command line add a -i option, e.g.

  ssh -i .ssh/id_dsa_host_1 host_1
  

  If you have a key for each server you can add the info in the .ssh/config file (see man ssh_config for details), e.g.

  Host host_1
  IdentityFile ~/.ssh/id_dsa_host_1
  
  Host host_2
  IdentityFile ~/.ssh/id_dsa_host_2
  

  Warner : `ssh-agent` can also be used to add multiple keys.
  From Dan Andreatta