When working with git, I have mainly used a single SSH key pair, which I use for e.g. GitHub.
Today, though, I had to add a second key pair to be able to clone another remote repository. Since I have not used multiple SSH keys simultaneously before, I was not sure how to define which key to use where.
Luckily, this great article suggests using an SSH config file, in which you can specify which ssh key to use where, and if you want to use a specific IdentityFile.