You can move up and down in Gmail using the j and k keys. These shortcuts work on Gmail If you have enabled keyboard shortcuts in Gmail, you may have noticed that pressing the J key selects the previous (older) message in your inbox while the K key moves you one message up to a newer conversation.

The same j & k keyboard shortcuts can be used for navigating items in Google Reader, for moving between tweets on Twitter.com, and it even works with Tumblr.com.

So why do developers commonly assign the k and j keys for moving up and down respectively and not any other key combination?