Swagelok Double Ferrule Compression Fittings
Swagelok is a particular brand of fittings. Their claim to fame is their "Double Ferrule" compression fitting, which uses two separate components for the ferrule. The design is essentially a double-ferrule arrangement, wherein the backup ferrule wedges in against the primary ferrule, the latter making up the sealing interface between the primary member (the tubing) and the fitting body, and the former effecting the seal between the ferrule and the tubing itself.
It is an effective and excellent design.
Refer to their website at https://nederland.swagelok.com/en/double-ferrule-compression-fittings.