Plain is related to Obvious. Obvious is a synonym of Appear. 
Relief is a type of Sculpture. Relief is related to Welfare. Welfare is related to Government. Government is related to Empire. 
Plain <--RelatedTo--> Obvious <--Synonym--> Appear 
Sculpture <--IsA-- Relief <--RelatedTo--> Welfare <--RelatedTo--> Government <--RelatedTo--> Empire 