Much of our knowledge on the subject of the mechanical properties of paper is of a technical nature, and though valuable to the purposes for which it was designed, relatively little of it is of fundamental significance. Discussed in this section are the observed and hypothetical phenomena that occur in paper during mechanical loading, and hence the effort is on the presentation of observations and ideas that relate to the development of theories of the mechanical deformation of paper. Although the emphasis is on properly designed mechanical studies of paper, one must remember that any hypothesis or theory must stand the test of all legitimate laboratory data, so that the mass of technical literature must be regarded as important.