Description of disordered structures has always been a big concern for crystallographers and for alloy researchers. Since alloys have specific chemical compositions, certain chemical building blocks should be present, on which the compositions reside. It has been long accepted that short-range-orders dominate many of the materials performance. Then the apparent composition rules should be routed in the chemical building blocks. For this objective, our cluster-plus-glue-atom model will be introduced, which regards any structure as being composed of a nearest-neighbor polyhedral cluster part and an outer-neighbor glue atom part situating between the clusters, expressed with universal cluster formulas [cluster](glue atom)x. Averaged chemical building blocks can be defined, mimicking molecules for chemical substances. Such local units provide the key clue towards understanding the seemingly complicated composition rules of all kinds of alloys, including quasicrystals, amorphous alloys, eutectics, many industrial alloys and high-entropy alloys.