Split tension mattresses explained: one bed, two firmnesses, no seam

The most common argument about a mattress is not the price. It is that one of you wants it firmer. Sixteen stone and a bad back on one side, nine stone and a preference for sinking in on the other, and a single spring unit that can only be one thing. Split tension is the traditional trade's answer, and it is a lot simpler than it sounds.

What split tension is

A mattress with two spring units side by side under one shared comfort layer. One side is built to a Medium tension, the other to a Firm. The comfort layers, the memory foam or latex or fibres on top, run across the whole width as normal, so there is no ridge, no seam, and no line down the middle of the bed. You feel your own support underneath and the same surface as your partner.

It is different from a zip-and-link, which is two complete single mattresses zipped together, seam and all, and which you buy mainly for getting up narrow stairs.

How tension is made

Tension is about the springs and the base, not the top. A Firm tension typically means:

• A heavier spring wire, 2.0 mm instead of 1.8 mm, so each spring pushes back harder
• A denser base foam under the springs, around 40 kg/m³ instead of 33 to 35
• A firmer encapsulation rail around the edge

None of that changes the memory foam or latex on top. Which is why, on a split-tension mattress, both of you get the same feel and different support. It is also why tension is essentially free on a made-to-order mattress: the factory swaps a wire gauge and a foam grade, and nothing else changes.

Which sizes

King (150 × 200 cm) and super king (180 × 200 cm). A double is too narrow for two spring units to be worth doing; each side would be 67 cm, less than a single. In a super king each side is 90 cm, a full single width, which is where split tension works best.

Who it is for

• Couples with a big difference in weight. A spring unit that is right for sixteen stone is too hard for nine stone, and the other way round.
• One back that needs firmer support, one that does not.
• A front sleeper and a side sleeper. Front sleepers generally need firmer support to keep the hips from sinking; side sleepers need the shoulder to settle.
• Anyone who has been sharing a compromise mattress that neither of you chose.

Who it is not for

If you both sleep the same way and weigh roughly the same, you do not need it. And it does not solve the other big couples' complaint, feeling your partner move, which is a job for the spring type and the comfort layer, not the tension. Pocket springs, where each spring is in its own fabric sleeve, carry far less movement than open coils; a foam core carries almost none. If that is your problem, look at our guide to mattresses for couples who move differently.

What it costs

At the traditional end of the trade, split tension is often free or a small surcharge, because the mattress is being made to order anyway. On boxed or rolled mattresses it is not available at all, since the mattress has to be made in bulk and compressed. We charge a small extra for the second spring unit and offer it on The Bad Back Years and The Hotel Bed in king and super king.

Because a split-tension mattress is made to your specification, it is exempt from the 14-day distance-selling cancellation right. Any honest seller will tell you that before you order. Faults and guarantees are unaffected.

Choosing your sides

Medium is right for most people, including most people with a sore back; the surface is the same either way. Go Firm on a side if that person is over about sixteen stone, sleeps on their front, or already knows they like a solid bed. If you are unsure, take Medium on both sides and let the 30 nights decide: a Firm can be softened with a topper, but a Medium cannot be firmed up.

The short version

• Two spring units, one comfort layer, no seam.
• Same feel on top, different support underneath.
• King and super king only.
• For weight differences and different backs, not for motion transfer.