To many people, however, the ‘obvious’ mechanism is not Cap & Share but either a carbon tax (discussed below) or a version of cap and trade applied ‘downstream’ where the emissions take place. Such a cap and trade system has two parts, as follows. The first applies to the fossil fuels we buy directly (petrol, gas, coal) and burn ourselves, causing emissions; these direct emissions account for half of our ‘carbon footprint’. For these direct emissions, some form of personal carbon trading is envisaged, typically based on ideas of ‘rationing’ familiar from petrol and food rationing during the Second World War. Personal Carbon Allowances (PCAs) typically involve giving an equal allowance to each adult citizen, and each purchase of petrol, oil or gas is deducted from the allowance (typically using swipe card technology). The other half of our carbon footprint consists of indirect emissions, the ‘embedded’ emissions in goods and services, which arise when companies produce these goods and services on our behalf. These indirect emissions are controlled with an Emissions Trading System (ETS) for companies