The SL3 is the new model. Then there is now some trickle down. So the old SL2 is now the new Pro (I think).

So in some cases, you have totally different bikes. And in some cases, it's just a difference in layup between models.

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The 2008 S Works Tarmac SL uses the same carbon (Specialized's 10r) as the 2009 and 2010 Pro SL. The 2008 SL2 has 11r carbon, the more hi-mod version, as does the 2009 and 2010. In a nutshell, the '09 and '10 Pro bikes have the same frame layup as an '08 S Works SL. Also different construction techniques between models/years.