An underlay, also known as a mattress pad or bedding underlayment, is a layer placed between a mattress and a fitted sheet. Its primary purpose is to provide additional comfort, support, and protection for the mattress. Underlays can help extend the life of the mattress by preventing wear and tear, as well as offering a barrier against dust mites and allergens. Underlays come in various materials, including memory foam, latex, and cotton, each offering different levels of cushioning and support. Some underlays are designed to enhance temperature regulation, while others may provide extra firmness or softness, catering to individual sleep preferences.