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Bulk metallic glass (BMG) formations in Co- and Fe-based alloy systems are investigated by using our cluster line approach in combination with minor alloying principle. Basic ternary alloy compositions in Co-B-Si, Fe-B-Y, and Fe-B-Si systems are first determined by cluster lines defined by, linking special binary clusters to third elements. Then the basic ternary alloys are further minor alloyed with 3 to 5 at.% Nb to improve glass-forming abilities (GFAs) and phi 3 mm BMGs are formed in (Co(8)B(3)-Si)-Nb and (Fe(8)B(3)-Y)-Nb but not in (Fe(8)B(3)-Si)-Nb, TM(8)B(3) (TM = Fe, Co) being the most compact binary cluster. The BMGs are expressed approximately with a unified simple composition formula: (TM(8)B(3))(1)M(1), M = (Si, Nb) or (Y, Nb). Finally, mutual Fe and Co substitutions further improve the GFAs as well as the soft magnetic properties, e.g., I(s) reaching 0.98 T and H(c) < 6 A/m for the Co-Fe-B-Si-Nb BMGs. Using the (cluster)(1)(glue atom)(1) model, a new ternary BMG Fe(8)B(3)Nb(1) is obtained.