Hi Kelly, There a few things that separate the 909 from the 875. The 909 was intended to be a "bigger" shoe with more support in the upper than the 875 as a result of the N-Lock webbing system, more overlays and an asymmetric design that provides added medial support. To go with the added structure of the upper, the soleunit of the 909 features 2 layers of foam. The top layer is a low density, high cushion foam. The bottom layer (red in the photo) is a higher density, less cushioned foam that is stiff enough to provide forefoot protection, midfoot stability and bilateral heel support. The 875 doesn't feature any additional support features as it is intended for a runner with a neutral gait. With all the added features, the 909 is intended for a runner looking for some added stability. The trade-off is weight though as the 909 does weigh more but you get added protection and support that provides some runners with added confidence on more aggressive terrain. Hope this helps. Please let me know if you have any more questions. - Bryan Gothie, New Balance Product Manager