This third method of construction uses different materials or levels of firmness in different parts or "zones" of a mattress support layer. It is also a way of combining comfort (pressure relief) and support (spinal alignment) in a single layer similar to a progressive construction. While it is often used in more "difficult" circumstances, some zoning schemes are poorly thought out in manufacturing or poorly understood by the person selling them and "suggested" when they are not appropriate. In these cases they they can do more harm than good. Some zoning schemes are also so similar in ILD or firmness between zones that they are not as effective as a higher quality material without zoning. While zoning when done correctly can be very beneficial, it is also the subject of some misinformation and deceptive sales practices which are used to sell mattresses. If you choose a zoned mattress, 2 or 3 zones are generally the most important and more than this can have questionable benefits and can sometimes lead to the purchase of an inappropriate and very uncomfortable mattress.
The reason for zoning is because of the difference in weight, thickness/thinness, and surface area of three main areas of the body that are the most difficult to accommodate. The first of these is the hip/pelvic area which carries the greatest amount of weight and has the greatest density, no matter what type of body you have, and often has a wider profile (especially in women). The second is the waist/lumbar area which is often lighter and thinner and has a lower density. The third is the shoulder/chest area. The shoulders are narrower and have less surface area (on the side) and will sink in more easily until they reach the torso which is often wider than the hips but lighter and lower density. Dealing with these 3 areas in all the different sleeping positions and differences in body profiles and the difficulties associated with balancing them in terms of pressure relief and alignment is the reason that zoning is sometimes used in more difficult or extreme body weights or profiles. It is appropriate and can be very helpful when a progressive or differential method of construction cannot accommodate the circumstances.
Three zone mattresses with a firmer layer in the middle under the lumbar and pelvis and 2 softer zones above and below this.
This is most commonly used to help correct alignment and is the most common type of zoning you will see. It uses "normal" ILD's in a support layer with the exception of a firmer layer in the middle. It is most useful when everything is fine in terms of pressure relief and support in all areas of the body except the hips are sinking in too deeply. This is usually a body profile with thinner and relatively heavier hips. This can be very beneficial since the hip area is the heaviest of the body and if they are thinner and the support layers are soft enough to allow the wider shoulders to sink down into alignment, then the hips in turn may easily sink down too far for correct alignment. Men in particular with wider shoulders and chests than hips may do well with this. Those with wider hips but more normal weight will likely not do well with this zoning scheme as their hips need to sink down further in proportion to the shoulder to accommodate their width and help with alignment.
This type of zoning is used to give better support to the lumbar and pelvic area.
Multi zone mattresses with a softer layer under the hips and shoulders and a firmer layer under the lumbar.
This is what we call "reverse zoning because the areas of softness and firmness have been reversed. This type of zoning is most appropriate for those who need to sink in more deeply with their hips and shoulders so that a firmer layer (or zone) underneath the recessed lumbar curve can "move up" and be in firmer contact with the lumbar area which is not getting enough support and tending to "collapse". This can either be because someone is too light and not sinking in deeply enough to their comfort layer leaving "air" or "non resilient foam" under the lumbar or because someone carries a great deal of excess weight around their middle and particularly on their back this weight is tending to collapse the lumbar and cause misalignment. While in these two cases where other solutions do not work this zoning scheme can be beneficial, it is often sold to someone who will not benefit from it and in these cases it can cause discomfort of the lumbar area.
This type of zoning is used to give better support to the area under the lumbar curve..
Two zone mattresses with a softer upper zone from the lower ribcage upwards and a firmer lower zone from the same point down to the bottom of the mattress.
This type of zoning is perhaps the least used. Because they have a smaller surface area than the hips, the shoulders will often sink in easily until the wider chest area comes in contact with the mattress comfort layer. At this point the chest which has more surface area than the hips and is also lighter will stop sinking if the comfort layer is not soft enough to allow it to sink in. This can lead to either alignment problems if the upper body is being supported in a position that is too high and out of alignment or pressure issues if the upper body and shoulder cradle is not deep enough. This can also lead to the lumbar gap not being filled in firmly enough to support the lumbar, upper back issues if the alignment there is not correct, or neck issues if it is out of alignment from the shoulder area being too high. By using a softer layer from the lower rib cage area to the top of the mattress, these problems can be solved. This would be of particular benefit for those with larger shoulders or chests that are bigger or wider than normal in relation to the hips. This type of zoning is often used in both the comfort layer and/or a middle layer immediately below it unlike the other two which are more commonly, although not always, used in the lower layers.
This type of zoning is used for pressure relief in the upper body and to help with upper body alignment.
One other "zoning" option that is sometimes used in a mattress support core are firmer coils or firmer foam around the edges to prevent "roll off" and to make them firmer for those who often sit on the edge of a mattress. In the case of some coil constructions, it is also done to keep the coils together and in also in the case of some specially constructed innersprings which are made to be used on an adjustable bed and do not have a thicker wire (called a border rod) around the edges so they can bend. High quality foam support cores such as latex do not need firmer foam around the edges and it can actually be a disadvantage as the edge foam will wear out long before the latex and can lead to "collapsible" edges and sagging over time. Otherwise this type of "zoning" would be based on preference.
Because zoning can be complex and may not be necessary at all, it should be approached with caution, especially if it is possible to accomplish the pressure relief and alignment you need without it. Zoning schemes that use more than 3 zones may also be less effective in some cases because the zoning differences are sometimes less than the natural differences across the surface of the layer (especially in the case of zoned foam layers) or the zones may not "fit" the specific body type or sleeping style of the person on the mattress. In some cases they can also be a justification for higher prices than necessary. In the end ... how well a mattress provides the pressure relief and neutral alignment you need in all your sleeping positions in your "real life" experience is a more important guideline than any "theoretical" benefits of a zoned design even though a zoned design can certainly be useful in the right circumstances and in a design that "matches" the needs and preferences of the person that would be sleeping on it.