Mechanic is related to Doctor. you are likely to find Advice in/at/on Doctor. Advice is a synonym of Tip. 
Theory is a different concept from Practice. Convention is a type of Practice. Conventional is derived from Convention. Conventional is a synonym of Typical. 
Mechanic <--RelatedTo--> Doctor <--AtLocation-- Advice <--Synonym--> Tip 
Theory <--DistinctFrom--> Practice <--IsA-- Convention <--DerivedFrom-- Conventional <--Synonym--> Typical 