A tax leads to a deadweight loss because it creates inefficiency: some mutually beneficial transactions never take place because of the tax—namely, the transactions Q_E – Q_T. The yellow area here represents the value of the deadweight loss: it is the total surplus that would have been gained from the Q_E – Q_T transactions. If the tax had not discouraged transactions—had the number of transactions remained at Q_E because of either perfectly inelastic supply or perfectly inelastic demand—no deadweight loss would have been incurred.