What is the difference between Resource Block (RB) and Physical Resource Block (PRB)?

I think I’m a bit confused with RB and PRB in LTE still. Reading the explanation of PRB - “The LTE PRB (Physical Resource Block) is made up of 12 consecutive subcarriers for a duration of one slot (0.5 ms)” this would mean that for 1 ms (1 Subframe) there are 2 PRBs or the same PRB but duplicated? Because if it is 2 PRBs for 1 ms that would mean that for 20 MHz bandwidth there should be 200 PRBs not 100 PRBs (Guard Band part here).
What am I getting wrong?