Which is NOT a standard RAID level?

Which is NOT a standard RAID level?

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EXPLANATION

RAID 2, which is rarely used in practice, stripes data at the bit (rather than block) level, and uses a Hamming code for error correction. The disks are synchronized by the controller to spin at the same angular orientation (they reach index at the same time^{[clarification needed]}),
so it generally cannot service multiple requests simultaneously.^[16][17] However, depending with a high rate Hamming code, many spindles would operate in parallel to simultaneously transfer data so that "very high data transfer rates" are possible^[18] as for example in the DataVault where 32 data bits were transmitted simultaneously.
With all hard disk drives implementing internal error correction, the complexity of an external Hamming code offered little advantage over parity so RAID 2 has been rarely implemented; it is the only original level of RAID that is not currently used.