Extend is a synonym of Cover. Ridge is a type of Cover. Ridge is related to Trail. Slot is a type of Trail. 
Tooth is part of Gear. Reverse is a type of Gear. 
Extend <--Synonym--> Cover <--IsA-- Ridge <--RelatedTo--> Trail <--IsA-- Slot 
Tooth --PartOf--> Gear <--IsA-- Reverse 