1 Boxspring
- serves as the foundation of the bed.
- provides the firmness to the mattress
- ideal pair for the mattress, they should always go together

2 Innerspring Unit
- how the coils are constructed is necessary.

3 Edge Support
- allows you to maximize sleeping space
- must be sturdy enough so you don’t roll off or flatten it down

4 Upholstery
- comes in variety and different quality
- makes the mattresses firm

via Better Bedding