The new concept of M-structures is used to unify and generalize a number of concepts in Hadamard matrices including Williamson matrices, Goethals-Seidel matrices, Wallis-Whiteman matrices and generalized quaternion matrices. The concept is used to find many new symmetric Williamson-type matrices, both in sets of four and eight, and many new Hadamard matrices. We give as corollaries "that the existence of Hadamard matrices of orders 4g and 4h implies the existence of an Hadamard matrix of order 8gh" and "the existence of 'Williamson type matrices of orders u and v implies the existence of 'Williamson type matrices of order 2uu". This work generalizes and utilizes the work of Masahiko Miyamoto and Mieko Yamada. Lists of odd orders < 1000 for which Hadamard and Williamson type matrices are known are given.