As an alternative to CDM and JI projects, emissions can be offset directly by buying and deleting emissions allowances inside the ETS. This is a way to avoid several problems of CDM and JI such as additionality, measurement, leakage, permanence, and verification. Buying and cancelling allowances allows to include more emissions sources in the ETS (such as traffic). Furthermore, it reduces the available allowances in the cap-and-trade system, which means that it reduces the emissions that can be produced by covered sources. The EU is negotiating a link with Switzerland's domestic trading system. Distinct emissions trading systems can be linked through the recognition of emissions allowances for compliance. Linking systems creates a larger carbon market, which can reduce overall compliance costs, increase market liquidity and generate a more stable carbon market. Linking systems can also be politically symbolic as it shows willingness to undertake a common effort to reduce GHG emissions. Some scholars have argued that linking may provide a starting point for developing a new, bottom-up international climate policy architecture whereby multiple unique systems successively link their various systems. "Official pages"  "How ETS works"  "Key reports, and assessments"  "Case law"