What is B-Key and M-Key in SSD?

SSDs come in different form factors. Form Factor determines the dimensions of an SSD. So, consumer SSDs mainly come in two form factors, i.e., SATA 2.5″ and M.2 Form Factor. M-Key and B-Key are related to the M.2 Form Factor.

Below are the M.2. and SATA 2.5″ form factors and their approximate dimensions.

M.2 is a compact SSD form factor, and because modern devices are getting smaller yet faster, it integrates perfectly with contemporary data transfer protocols like PCIe and NVMe.

Although M.2 isn’t limited to NVMe SSDs only, it is most commonly used with NVMe drives. M.2 SATA SSDs are also available in the market.

Therefore, there are two types of SSDs that utilize the same form factor. One is SATA (slower), and the other is NVMe (faster).

An M.2 SATA SSD appears as follows.

An M.2 NVMe SSD appears as follows.

Saw any difference?

Both SSDs shown above are 2280 SSDs (22x80mm). So dimension-wise, you can see no difference. But the notches are different. One is using a B+M Key, and the other one is just an M-Key.

B-Key, M-Key, and B+M Key SSDs

Now, there is a rule that NVMe SSDs will always have an M-Key (a notch on the right side), and M.2 SATA will have a B+M Key (two notches on both ends). You may also find some SSDs with B-Key (Left Notch), but they are rare these days.

M-Key

• Position: The notch is on the right side when the SSD label is facing up and the gold contacts are away from you.
• Interface support:
  • Can use PCIe x4 lanes (4 lanes of PCI Express)
  • Typically NVMe only, much faster than SATA
• Used in: Most modern high-speed NVMe SSDs
• Socket type: M.2 Socket 3

B-Key

• Position: The notch is located on the left side (viewed with the label facing up and the gold contacts facing away from you).
• Interface support:
  • Can use PCIe x2 lanes (2 lanes of PCI Express)
  • Can also use the SATA interface
• Used in: Some cheaper NVMe drives and most SATA M.2 drives
• Socket type: M.2 Socket 2

B+M Key

• Has both notches.
• Designed for maximum compatibility, but with a limitation — it can only use PCIe x2, even if the slot supports x4.
• You’ll often see this on budget drives or SATA-based M.2 SSDs.

Conclusion

Modern M.2 NVMe SSDs always come with the M-Key notch while the M.2 SATA SSDs will generally have the B+M Key design. The B-Key M.2 SSDs are rare to see these days. The M.2 standard itself is defined by the PCI-SIG (Peripheral Component Interconnect Special Interest Group), in cooperation with the SATA-IO (Serial ATA International Organization) and other groups under the NVM Express Working Group. These bodies determine the mechanical dimensions, electrical pinouts, and keying positions, which specify exactly where those notches are located and what interfaces each one supports (e.g., SATA, PCIe x2, PCIe x4, USB, etc.). The idea behind the keying is to physically prevent you from plugging incompatible hardware into a slot. For example, a SATA-only M.2 drive in a PCIe-only slot. So, if you are curious who decides these notches and dimensions, it is mainly the PCI-SIG.