In a crystalline substance, the particles of a substance occupy an ordered position in relation to each other. The folded chains then coexist in the folded state. Since plastics are chain molecules, they are difficult to structure. Consequently, most plastics are (partially) amorphous. There are three conditions under which a plastic can crystallise: the chain must be linear (i.e. it must have no branches), the side groups must not be bulky and the chain must have a regular arrangement of the side groups relative to the main chain (tacticity). However, complete crystallisation is not possible in macromolecular substances. This is because the chains never show a completely regular structure and entanglements occur between the chain molecules that disrupt the crystal pattern. Thus, in addition to the crystalline phase, an amorphous phase always occurs in a plastic. Consequently, crystallinity is expressed as the percentage of crystallised material relative to the total polymer.