Crossword puzzle clue "molars usually have four of these"

Molars usually have four of these

These usually have two channels

Collection consisting of four plates, four saucers, four bowls, etc.

They usually have four burners

What words that are fun to clue in crosswords usually have ... and what appear in each of this puzzle's four corners

They usually have four strings

Anyone with molars, e.g.

Shiny metal replacements for molars: 2 wds.