The term 'MATER' (Brain Cover) can be broken down into two crossword solutions: PIA (3 letters) and DURA (4 letters). PIA stands for the pia mater, which is the innermost layer of the meninges, a protective membrane that covers the brain and spinal cord. The dura mater is the outermost layer of the meninges and is composed of dense connective tissue. Together, the pia mater and dura mater form the MATER, or brain cover.