B is for on their BACK.

Babies should always be laid on their back to sleep.

If a baby is laid on its stomach or side, it is very easy for them to roll forward with their face in the mattress. Babies do not have the neck strength to turn their heads away from the mattress to keep a clear airway.

When a baby starts rolling over on its own, you should still initially lay the baby down on its back, but at that point they will have the muscle strength to roll onto their bellies and move their heads on their own.