Coins

 are the currency of Terraria.

Obtaining coins
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Coin types
Coins come in different types. They can be crafted to and from other coin types without loss of value. It takes 100 coins to upgrade, and you get 100 coins of lesser value when one is "broken" or downgraded.

Curiously, Platinum is not a mineral that can be found in ore/bar form, making it a mystery as to where the coins come from. Platinum is the most valuable coin available.

Death penalty
Upon death (in Softcore Mode), half of all the stacks you have of coins in your inventory will be dropped (rounded up if the number is odd). For example, if you have 1 Gold coin in one stack, you will lose it, but if you have 12 coins you will lose 6. You will still lose half of your coins if you commit suicide with self-thrown explosives.

Storage
Coins have their own storage slot but can also be stored in the normal inventory like anything else. Coins can be stored in a Chest, Safe, Piggy Bank, Barrel and any other form of storage. Coins placed in any stationary container will not be lost when you die. It is advisable to place all your coins in a Chest, Safe or Piggy Bank every time you return to your home or before fighting a boss.

Bugs

 * It was possible to keep one stack of coins on death by "holding" it with your mouse arrow right before you die, but as of the patch 1.1.1, mouse items drop normally.
 * By shift-clicking your coins while shopping with an NPC you will lose all the coins you clicked, and all the coins you sold them for.

Trivia
Coins are one of the few things that can be sold and bought for the same price; because of this, you can make it look like you have an abnormally large amount of money above your head, but the look is purely aesthetic. To do this trick, you must simply sell the coin and rebuy it from the same slot quickly.

Also, a good way to get a lot of coins is to go to the corruption with some armor and good supplies and potions, and kill Eater Of Worlds until you get a lot.