Extra Firm Mattresses usually refer to mattresses with a perceived hardness of 9-10 degrees. This type of mattress usually contains a soft layer such as memory foam that is not very thick, usually between 2-3cm, and then a harder foam or a combination of hard foam and springs underneath. Sleeping or sitting on an extra firm mattress, you will feel strong support and will not sink too much. If you like this feeling, you can choose the extra firm mattress.