A box spring is a type of bed base that is designed to support a mattress and elevate it off the ground at a comfortable height. It is typically made up of a sturdy wooden frame covered in cloth and containing springs. The box-spring is usually the same size as the mattress that is placed on it, and together with the optional bed frame, they make up a bed. The purpose of a box-spring mainly lies in the trade-off between ventilating the bed through the bottom and structural stability of the box-spring itself. Some of the benefits of using a box spring include:

• Height: Box springs add height to the bed, making it easier to get in and out of bed.
• Sturdiness: Box springs give mattresses an even and flat surface to rest on, which helps to prevent sagging and uneven wear.
• Protection: Box springs can help to protect the mattress from damage by providing a barrier between the mattress and the bed frame.
• Ventilation: Box springs can help to improve airflow and breathability, which can help to regulate temperature and prevent the buildup of moisture.

Box springs are typically placed on top of a wooden or metal bed frame that sits on the floor and acts as a brace, except in the UK where the divan is more often fitted with small casters. It is common to find a box-spring and mattress being used together without the support of a frame underneath, the box spring being mounted directly on casters standing on the floor. However, box springs are not always necessary and may not be suitable for all types of mattresses. For example, foam mattresses do not require a box spring and may even be damaged by using one. In such cases, alternative options like platform beds and wood slat foundations can be used instead.