CERs were fungible with the EU’s permit-based Emission Trading Scheme (EU ETS), until both systems struggled with excess supply, resulting from over-permitting in the EU ETS and low targets under the Kyoto Protocol. As part of the solution for this, CERs have not been permitted in the EU ETS for many years. The EU scheme has now been overhauled, leading to record high prices and the CDM is due to be transformed under Article 6 of the Paris Agreement, which enables trading between nations. The latter is currently limited to pilot programmes under bilateral agreements but is expected to grow and follow many of the best practices - and indeed standards - developed in the voluntary carbon market. Increasingly, other compliance schemes are utilising the voluntary market, the closest example being CORSIA, the global aviation sector offsetting scheme.