Yags was first conceived during my final year at university, sometime towards the end of 1994 and beginning of '95. At that time, I was using my Myths system to run campaigns, but my tastes had changed and I decided I wanted to start afresh. Myths was getting too complex to extend (character packages in particular meant there was a lot of work involved in adding a new setting), and I wanted something simpler. And so Yags was born.
Version 0.1 (Taneith)
The first version of Yags to be played was written as a fantasy game for my Taneith setting. It was far simpler than the most recent incarnation, and attributes and skills were added together for skill checks. Attributes averaged to 0, and skills were in steps of +5, adding 2d10 to get a result.
Version 0.2 (Habisfern)
This was the first version to use multiplication of attribute and skill to achieve a result. However, the skill was a percentage, in steps of 25%. This required a bit of arithmetic, though the design of the character sheet turned that into a simple lookup.
The multiplication was added so that neither skills nor attribtues could dominate. I like both affecting ability, but with addition I've found it difficult to balance how much effect each has. By multiplying, little or no skill will tend to negate the advantages of a high attribute, but at higher skill levels the attribute becomes more important.
Version 0.3 (Babylon 5)
This version was pretty much the same as 0.2, but with added rules for modern/SF combat and Psionics, to allow the running of a short lived Babylon 5 campaign. The move to modern was relatively painless.
Version 0.4 (Habisfern)
This was the first simplified version, again focused on my fantasy campaign. Attributes averaged at 3, and skills at 4, and were simply multiplied together. Sometime during the running of this campaign, the +2d10 became +1d20. I liked 2d10, since it made things more predictable but the players wanted a bit more randomness.
Version 0.5 (Harn)
Taking the Harn background, I modified Yags to fit a more structured character generation system, with backgrounds providing defined sets of skills.
This had the big advantage that character types that needed large numbers of skills for background reasons could have them without becoming unbalanced (most nobles should have skills such as heraldry, etiquette etc, but in a points-buy system taking such skills makes them less effective at 'adventuring' than a peasant foot soldier who can concentrate on combat skills).
It also added rules for magic and religion.
Version 0.6 (Habisfern/Unplayed)
Backporting the Harn changes to my Habisfern setting become the 6th version of the rules. This introduced Techniques, and also reminded me why I didn't like have character packages - designing a good selection takes a lot of time and effort, and always ends up restricting the type of character the player can create.
Though this version was never actually played, it was a testing ground for a lot of ideas that finally made it into the next revision.
Version 0.7 (Traveller)
This is the latest version of the rules, and has caused the focus for the base system to be changed from fantasy to modern day.