When you collect commerce by working tiles or having commerce-producing buildings (such as the palace), you can spend them on research, or set them aside as gold. As the game progresses, you also get the options to spend your commerce on espionage and culture. However, if you choose to set aside your commerce as rainy day funds, you can't spend it on research, espionage, or culture. You can spend it on military unit wages, civic upkeep, or city maintenance. If you have universal sufferage, and, barring everything else, you can use it as trade fodder. But, I personally would like to see a mod that lets you re-convert your gold back into your choice of research into the tech of your choice, espionage against the enemy of your choice, or culture pumped into the city of your choice. Imagine being able to ... spend 10 gold and have your new city expand into the BFC immediately. Learn a new tech instantly. Immediately get a Great Spy's worth of espionage points, so you can create about half a dozen city revolts right as your soldiers are right outside their gates, saving you the need to bombard their cities and being able use ALL your seige weapons for collateral damage instead of bombardment. Anybody else think this would be a good idea?