Historically, softer mattresses were associated with sagging and therefore bad backs over time. By building body zones with resilient materials into our mattresses, we've been able to better contour your body's overall shape and provide more support where it's needed (like in your hips and lower back), and give where it's more important (like your shoulders). This is why you'll see holes or cut-outs in our all-foam mattresses, and why the springs in our hybrids compress to different degrees. Where mattress firmness options are provided, this is often more a matter of subjective preference than impacting overall comfort or ergonomics. Those who prefer a huggier, enveloping feel should select a medium mattress, while those who prefer to sleep above the mattress surface should select the firm option. For the product lines in general: 1. The One is considered quite firm with a very minimal hugging feeling at the top layer. 2. The Original is generally considered a medium-soft mattress that has the most hug-feel (without being sinky). 3. The Original Hybrid is considered a medium mattress that's more balanced between the One and the Original (with most preferring this). 4. The XX is most similar in laying feeling to the Original Hybrid, with the topmost layer adjusted for a bit more of a cuddly cloud feel, so would be considered a bit softer (but still firmer than the Original)."