Elder Scrolls: How often do Elder Scrolls games come out?

My son bought me Elder Scrolls V: Skyrim around Christmas time. I've been hooked ever since. I'm not familiar with the series and just wanted to know if it was a yearly release or every other year? I really doesn't matter because all of the content, but just wanted to know, thanks.Elder Scrolls: How often do Elder Scrolls games come out?
Here's the exact release timeline:

1994 The Elder Scrolls: Arena
1996 The Elder Scrolls II: Daggerfall
1997 An Elder Scrolls Legend: Battlespire
1998 The Elder Scrolls Adventures: Redguard
2002 The Elder Scrolls III: Morrowind
2002 The Elder Scrolls III: Tribunal
2003 The Elder Scrolls III: Bloodmoon
2003 The Elder Scrolls Travels: Stormhold
2004 The Elder Scrolls Travels: Shadowkey
2004 The Elder Scrolls Travels: Dawnstar
2006 The Elder Scrolls IV: Oblivion
2006 The Elder Scrolls IV: Knights of the Nine (Expantion Pack)
2007 The Elder Scrolls IV: Shivering Isles (Expantion Pack)
2011 The Elder Scrolls V: Skyrim

When interested in the history of the games, check Wikipedia:
http://en.wikipedia.org/wiki/The_Elder_S鈥?/a>Elder Scrolls: How often do Elder Scrolls games come out?
The games were released on these years:

1994

1996

2002

2007

2011