Abstract: The present invention relates to the technical field of therapeutic support devices, and more particularly relates to a novel foldable and portable support device for use in therapeutic applications, such as in providing support to the user's lumbar and other related parts of the human torso. The support device of the invention finds particular application as an underlayment for conventional type of commercial and/or domestic sleeping mattresses and it features a new and novel hinge arrangement to maintain the device, and hence the mattress, in a flat condition while in use.