Bar stools
like this only use a Flat Bar Stool Swivel 
Bar stools with a back
may have either a Flat or Tilted Bar Stool swivel 
How do you know whether your bar stool seat swivel is flat or tilted? Choose either method
- If stool is not disassembled, measure from under the seat to the floor at both front and rear. If seat front is higher, it is a Tilted Bar Stool Swivel
- With the seat or swivel removed, observe if both steel plate edges (on all four sides) are parallel to each other.
If they are the same distance apart you need a Flat Bar Stool Swivel.