Packing refers to the distribution of grains and intergranular spaces (either empty or filled with cement or fine grained matrix) in a sedimentary rock. It is controlled by grain size and shape and by the degree of compaction of a sedimentary rock; in turn it determines the rock's bulk density. A description of packing is generally based on the analysis of thin sections of a sedimentary rock using a petrographic microscope. Particular attention is paid to the number of grain to grain contacts (packing proximity) and to comparisons between the sum of the lengths of grains to the total length of a traverse across a thin section (packing density). The amount of fine grained matrix and the matrix grain relationship affect the packing and fabric of a sediment and are important in interpretations of depositional mechanism and environment. Where grains in a sediment are in contact, the sediment is grainsupported; matrix can occur between the grains, as can cement. Where the grains are not in contact, the sediment is matrixsupported.