Plain is a synonym of Usual. Usual is related to Regular. Regular is related to Quality. 
Piece is related to Watch. Watch is a type of Check. Check is related to Mark. Mark is a type of Evaluation. 
Plain <--Synonym--> Usual <--RelatedTo--> Regular <--RelatedTo--> Quality 
Piece <--RelatedTo--> Watch --IsA--> Check <--RelatedTo--> Mark --IsA--> Evaluation 