You can choose to add on edge rail support to your slab foam. Edge rail support consists of a much firmer foam which surrounds the slab on the edges to give you more support when you sit on the edge.
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Sizes:
Twin: 39" x 74"
Twin XL: 39" x 80"
3/4: 48" x 74"
Double: 54" x 74"
Queen: 60" x 80"
King: 76" x 80"