Was reading bearcarver's rib thread
would like to know what this means
~~Note: I decided on "2.5-2.5-1", because when I use 3-2-1, I never get Fall Off The Bone.
As Chef Willie said, it refers to time in hours but it's only a guide. For me, following the 3-2-1 for spare ribs has always worked out great. I keep my temps pretty tight around 225°F throughout the smoke and believe is the reason why this method works consistently.
Baby back ribs vary more in thickness thus, requiring adjustments to the 2-2-1 method mostly by adding time in order to have them smoke/cook the way I like 'em. Good luck and enjoy both the process and the results!