It is widely accepted that quarks and leptons should be understood on the basis of the same unification scheme. The investigation of hidden rules behind observed quark and lepton mass spectra will provide a very important clue to a unified model of quarks and leptons. Now the investigation is timely because of the recent abundance of data on the CKM matrix elements and neutrino mixings. This volume offers useful information and hints on a unified understanding of quarks and leptons.
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