Key Escrow
A key escrow is typically a third party that safely stores a copy of private keys. They use the M of N control. The M has to be greater than 1, and the N has to be greater than the M. For example, employing 5 trusted individuals (N) would require at least 2 (M), each having part of the key.
This is in case a private key is damaged or lost. Making multiple copies of the private key becomes challenging to manage and could lead to compromise if stored on organization media.
A company could implement its key escrow by having multiple trusted employees with part of the key. Two or three of them could have USB drives with part of the key.