Q-in-Q 802.1ad
Putting a VLAN tag outside the customer's own VLAN tag, so a carrier can transport a customer's entire VLAN space without caring what's in it.
An ordinary 802.1Q tag allows 4094 VLANs, which is plenty inside one company but useless for a carrier keeping thousands of customers apart, each of whom wants their own VLAN numbers. The answer is two tags: the outer one, with TPID 0x88a8, belongs to the service provider and identifies the customer, while the inner one belongs to the customer and passes through untouched. So a customer can use VLAN 100 without caring that their neighbour does the same. The price is four extra bytes per frame, requiring the transport network to handle an MTU slightly above 1518. At larger scale VXLAN and EVPN have effectively taken over the role.