Pores are void spaces which must be distributed more or less frequently through the material if the latter is called "porous".  Extremely  small voids in a solid is called "molecular interstices", very large ones are called " caverns." Pores are void spaces intermediate between caverns and molecular interstices. The pores in a porous systems may be interconnected or non-interconnected. The interconnected part of the pore system is called the effective pore space of the porous medium.

A textile material is regarded as an assembly of yarns. This assembly will be permeable to fluid flow because of the existence of spaces or pores between the yarns (inter-yarn pores). The yarns themselves will also be permeable to fluid flow because of the spaces or pores that separate the individual filaments in the yarn (intra-yarn pores). The total flow of liquid through a textile will therefore depend on the part played by both types of pore.

Backer found four types of major interstices based on the mode of yarn intermesh and produced simualated sectioning diagrams that correspond with theoretically slicing the fabric horizontally.