In the past, if you had back pain, many doctors and well-meaning friends probably directed you toward a firm mattress. As it turns out, that advice was wrong. What you really need, according to the peer-reviewed research, is a mattress that feels medium-firm to you.

The key word here is you. Firmness levels are highly subjective. Owner reviews reveal that the same mattress can feel too soft for some people and too hard for others. Apart from personal preference, such discrepancies exist because a person’s weight and shape can influence the feel of a mattress. On a soft mattress, for example, a 130-pound body might feel like it’s floating. But for someone over 200 pounds, that same mattress could feel overly firm, given that they would sink immediately to its bottom support layer.

Here’s the rule: When people with back-pain lie on a mattress they determine to be medium-firm, that mattress is the best backache-relieving mattress for them—regardless of how the company labels it or whether the mattress has “support zones,” reinforced layers, or any other bells and whistles that cater to a spine-focused buyer.

Mattresses labeled medium-firm are a good place to start because they’re likely to support your spine’s natural curves whether you sleep on your back, stomach, or side. Ahmed Radwan, PhD, DPT, co-author of a Sleep Health: Journal of the National Sleep Foundation review paper about mattress design and sleep quality, explained to us in an interview that although people might prefer to fall asleep in a particular position, they can shift a lot during the night. Thus, “the successful mattress is the one that accommodates many positions and helps you wake up with less pain and tenderness.”

The “medium” part of “medium-firm” cushions the heavier parts of your body in a supportive way without letting them sink too far into the mattress. This helps relieve any pressure points without allowing your spine to slip out of alignment. When you’re sleeping on your back, your shoulders and hips can settle into a medium-firm mattress just enough that your lumbar (lower-back) region is supported and remains concave. When you’re on your stomach, your belly is both cushioned and held up, rather than caved in and forcing your lower spine into the shape of a hammock. When you’re on your side, the mattress offers enough cushion to support your shoulders, hips, and knees, but not so much that your spine flexes or twists.

The “firm” part of medium-firm helps keep the spine aligned. If you were to take a picture of yourself in each of these three sleep positions (a good thing to do when you’re shopping in person, as we suggest in our guide about how to choose a mattress), your body should be roughly straight, in neutral alignment. This means back- and stomach-sleepers should be able to draw a line from the neck to the lumbar (lower spine) region to the knees, while side-sleepers should be able to draw a line along the midpoints of the ears, shoulders, and hips. (See the illustration of ideal spine support below.)

If the mattresses you’re interested in are available in a store, and you can shop in person, bring your favorite pillow and try the mattress out before you buy. (Our bed-pillow guide can help you find the best one for your preferred sleep position.) Spend at least 15 minutes lying on the beds and focus on your spine: Does it feel relaxed, rather than flexed or strained? At the same time, does your body feel cushioned, as opposed to pressed against the surface of the mattress?

If you already have a medium-firm mattress but are still experiencing back pain, your mattress could be too old. “Whenever patients tell me that their back pain is the worst first thing in the morning, one of the things I always ask is, ‘How old is your mattress?’ They often say, ‘I don’t know, maybe 20?’ or some big number. But we know that mattresses usually don’t last more than 10 years, depending on their materials,” said Marlís González-Fernández, MD, PhD, associate professor of physical medicine and rehabilitation at the Johns Hopkins University School of Medicine.

Research bears out those claims. A study published in 2008 asked 33 women and 29 men to record back and shoulder pain for 28 days, first in their own beds (which were, on average, nine and a half years old) and then in new beds of similar quality. The researchers found that the new mattresses reduced back pain by 62.8% and stiffness by 58.4%. “Over time, the mattress can soften and sag,” said Bert Jacobson, EdD, the study’s lead author and a professor of health and human performance at Oklahoma State University. “It becomes less good at aligning your spine.” Even if you start out with a medium-firm mattress, you might not actually be sleeping on one 10 years later.

Of course, we can’t know exactly what medium-firm feels like to you, so in this guide we’ve highlighted the models that we expect will feel medium-firm to most people. Whatever you decide, make sure to take advantage of trial periods and return policies. Ideally, you should test out a mattress for at least 60 days, said González-Fernández (our picks offer at least 100-day trials): “Trying a mattress for a long time is important and can help make sure that a mattress works for a particular person.”