The single layer and double layer are both great for babies up to 35 pounds. The single layer sling is light, cool and breathable. Most people prefer the single layer for newborns and smaller babies. Double layer slings are thicker and heavier. That thickness provides lots of support and comfort, especially for carrying larger babies for longer durations of time. Double layer slings take longer to break in but they get nice and comfy once they are. Read more about our them on our fabrics page.