The Marrakesh Accords of the Kyoto protocol defined the international trading mechanisms and registries needed to support trading between countries ("sources can buy or sell allowances on the open market. Because the total number of allowances is limited by the cap, emission reductions are assured."). Allowance trading now occurs between European countries and Asian countries. However, while the US as a nation did not ratify the protocol, many of its states are developing cap-and-trade systems and considering ways to link them together, nationally and internationally, to find the lowest costs and improve liquidity of the market. However, these states also wish to preserve their individual integrity and unique features. For example, in contrast to other Kyoto-compliant systems, some states propose other types of greenhouse gas sources, different measurement methods, setting a maximum on the price of allowances, or restricting access to CDM projects. Creating instruments that are not fungible (exchangeable) could introduce instability and make pricing difficult. Various proposals for linking these systems across markets are being investigated, and this is being coordinated by the International Carbon Action Partnership (ICAP).