The four types of electrode Tamil revision mind map presented as a two-by-two comparison grid: the metal-metal ion electrode such as copper in copper sulphate solution, the metal-insoluble salt electrode such as the silver-silver chloride reference electrode, the gas electrode such as the standard hydrogen electrode with inert platinum, and the redox electrode such as iron-three iron-two with inert platinum, each with its electrode notation and standard reduction half-equation, plus the shared rule that half-equations are always written in the reduction direction with electrons on the left.