The rules largely reduce the risk of double counting, but are not completely airtight. The main tool for avoiding double counting is to apply “corresponding adjustments”, which are required for all authorised carbon credits. Rather like in double-entry bookkeeping, this involves the country selling carbon credits to deduct them from its own greenhouse gas inventory so that the country (or airline) buying them can count them towards its own climate targets. This clearly signals that double counting will not be tolerated in the official system.   However, “voluntary” credits purchased by private companies do not have to go through the Article 6 system. This means that largely unregulated private schemes can still allow double counting, even though this defies logic and environmental integrity. It remains unclear whether buyers will even want double-counted credits when properly adjusted credits will be available.  In addition, the way in which these adjustments will be applied to some varieties of credits is problematic. This is primarily the case when it comes to accounting for ITMOs under CORSIA, the carbon offsetting scheme for aviation. Countries with a single year target for their nationally determined contribution (NDC) – such as reducing emissions by x% by 2030 – which is the case of most countries, will apply adjustments in the target year as an average of all credits sold and purchased over the entire NDC period. Take a country that sells 0 credit over the first nine years of its NDC period, and then sells 100 credits to an airline for compliance under CORSIA during the 10th and last year of its NDC period. The airline will count 100 credits, but the selling country will only correct for the average quantity of credits sold over its NDC period, i.e. 100/10=10 credits. 90 credits are double-counted.