where α(α01), α(α11), α(α12) and αtri(α01, α11, α12) are the 5-dimensional vectors. α01 is the center (major) amino acid, α11 and α12 are the left and right amino acid (subordinate) respectively. There is no location difference between α11 and α12, that means αtri(α01, α11, α12) and αtri(α01, α12, α11) are equivalence. In the third layer, the hierarchical clustering (Ward's algorithm [21]) is used to cluster 4200 trimers into 199 clusters [19,18]. All clusters constitute the dictionary.