Why does RAID-5 require an additional disk for parity blocks?

Question

I know that RAID-5 consists of block-level striping across multiple disks, but using an additional parity-check block on each disk .. and that at least two disks are required for striping.

And it's obvious that each parity block is specific to each disk it belongs to (and so there is no need for allocating an additional disk).

_{Image from Wikipedia.}

However I've been unable to understand why in fact there is an additional disk required for parity checks, as I found on this article:

The minimum number of disks in a RAID 5 set is three (two for data and one for parity).

Any idea?