It’s somewhat fashionable to label any number in a game a currency. In practise, however, it gets a little confusing.
Anyone can grasp the idea that in-game gold is a currency, but what about your character’s health or experience points? Advocates might say that the player is trading health for damage or progress during the course of the game. But to most people that’s pretty tenuous.
I prefer to think of currency as one type of resource instead. This is a post about different types of resources that you could use in your game (including currency) and some guidelines on how to use them well.
In videogames most variables are hidden. Hidden variables cover everything from movement speeds and damage to loot drop percentages and AI behaviour. To discuss them in their totality is therefore an enormous and highly technical discussion because every game contains hundreds, or even tens of thousands, of them.
However some variables are visible. The player sees coin totals, experience point bars, levels and other numeric quantities that tell them important information. They also see numbers of things, like how many soldiers or crops they own. These kinds of variable are resources.
Resources play a crucial role in most games. One of the most common win conditions, for example, is to have more of a resource than an opponent. Another is to take all of his resource away. Resources are used for building, trading, scoring, assessing damage and death. They help the player know exactly how far he has progressed, unlock features and extend actions in many ways.
Resources, rules and actions are the levers that comprise the frame of the game, and to the play brain the frame is the totality of what games are (but to the art brain games are much more than that). Four common types of resource are: currencies, meters, tools and territory.
A currency is any resource that the player acquires and retains for use during play. He might acquire it via earning, purchasing, a gift or a game rule that grants him possession. He can then attempt to trade, exchange or convert it to something else.
This normally means coins, cash, supplies and so forth. It also encompasses less obvious meanings such as a ball in a field sport or ammunition in a shooter. However not scores, experience points, levels or health (which are meters, see below). Games usually have at least one currency, and some have many.
The ball in NFL football is a unit of currency, and most of the play revolves around who has the ball and who uses it to score (trade for points). Starcraft has two currencies, crystal and vespene gas. Crystal tends to be used for more basic buildings, but vespene gas is used for research and more advanced structures. Supply, a third resource, is a meter.
Roleplaying and strategy games often have currencies, but separate them into primary and secondary types. CityVille has many secondary currencies, such as special building materials or collectible items. The player typically gifts or collects secondary currencies over time and only uses them occasionally. Marble is vital to complete certain community buildings in CityVille, but not needed constantly. Collections of corn items can be traded as sets for special bonuses, but the player doesn’t do that all the time.
The differences between primary and secondary currency are prominence and frequency of use. Primary currencies are usually few in number, obvious and constantly earned or spent. Secondary currencies tend to be hidden from view, belong to sets (like CityVille’s collectibles or health potions in Diablo) and are stored for occasional use.
A meter is any resource that increases or decreases through play as a result of player action, and whose values can cause conditional rules to trigger. In common understanding this means score, experience points, health, roleplaying attributes like strength and dexterity, levels and time. It does not mean gold, cash or supplies (which are currencies).
There are two differences between a currency and a meter. The first is that the player chooses to spend a currency. The second is that a player can amass a currency and choose not to use it. Whereas a meter automatically tracks player action and triggers consequences (good and bad).
NFL Football has four meters: Game time, the play clock, downs and points. The first two govern the clock and uses of it. Teams can spend time outs to temporarily halt the clock, but it will always tend toward zero. Downs govern whether a team is permitted to retain possession of the ball and exert a pressure toward scoring. And point totals at the end of game time determine who wins.
But why are time outs a currency, yet downs are a meter? Teams have to use downs as a part of play, so although they regenerate through acquiring possession of the ball or achieving more than ten yards, there is no choice. The team is not spending a down by snapping, the down is metering how many snaps they are permitted to have before losing possession. However the coach can store time outs and chose to use or not use them throughout a half.
In a similar vein, energy in social games is often labelled a currency, but is actually a meter. The player does not deliberately trade, exchange or convert energy into something else during the course of play, it just decreases as a part of performing actions (such as harvesting crops) that are obligatory. The player can’t really amass energy either because it is usually capped (like downs). Some games make a secondary currency out energy packs or batteries that players can gift to one another.
Meters commonly trigger conditional rules. Character death from a lack of health is one example, as is the end of game time in an NFL match (which combines with the points meter to determine who won). Reducing Doctor Robotnik’s health to zero at the end of a Sonic level is another. It ends the level and moves the player to the next one. Completing tasks and acquiring experience points is yet another, as is attaining enough score to get on a high scores table.
As with currencies, games that have many meters tend to group them into sets. Starcraft has dozens of meters, from the overall global tally of supplies (which restricts the players army size) to every unit’s energy, damage, and improved armour or weapons. However each unit has the same types of meter, so the player can assess many at once.
Another kind of grouping, commonly seen in roleplaying games, is to gather meters into sets by type. A player will have attributes, skills, health, armour or energy etc, which the game groups in the interface. Groups will also tend to have similar number ranges.
Roleplaying games also commonly use universal (or near-universal) systems for resolutions of interactions between meters. Percentage rolls against skill, for example, or adding attributes and skills versus a dice system, lets the player know that while there are many numbers to contend with, they are all sort of the same thing. This breeds confidence.
Suppose you’ve spent 500 crystals in Starcraft to build five Protoss zealots (a kind of solider). You order them to attack opponents. You may lose some of them along the way. They are clearly a resource, but are the zealots a currency that you spend, or a meter that might decline?
Neither. They are tools.
Tools are resources that extend the player’s ability take action in a game. They are tracked by the player (how many zealots do I have?). They often use meters or currency, like ammunition for a rifle, supplies for a restaurant or possession of the ball for an NFL offense. Some are mobile, such as an object that you can pick up, whereas others are fixed in the game, such as a switch to open a door.
Tools differ from currencies (such as a ball) because the player usually permanently retains possession of the tool, but not currencies. Currencies are often contested for possession. A snooker ball is currency, as is a football. A snooker cue is not, nor is a football player. A time out in NFL is not directly contested, but is often spent to relieve opposition pressure or the play clock, and when it's gone it's gone. Darts, on the other hand, are tools that the player uses repeatedly.
In CityVille, a player’s tools are the buildings, crops, ships and other objects that generate revenue, population and experience points. He invests in them, builds them and then collects from them.
Games with many tool classes tend to suffer robustness issues. As the player experiments with different kinds of tool, he will probably find an exploit (a combination that makes the game easy to win, and thus boring) because the game designer simply can’t balance them all.
Though it may appear that a CityVille player has lots of tool choices, there are only a few classes: housing, community buildings, businesses, supply generators (crops and ships) and decorations. Each has a basic loop that repeats consistently throughout the game, and each has many individual tools that have slightly different meter and currency requirements.
It’s the same in Starcraft, Grand Theft Auto, Halo and Mass Effect. Every car in Grand Theft Auto is the same car, just with different handling values. Every spell in Final Fantasy belongs to one of a few basic types, every character you speak to in Dragon Age is the same entity, and every fighter in Street Fighter 2 is a variation on the same base model. They have different damage values, button combinations and animations, but the process of resolving interactions is always the same.
Ownership of location is important in many games. Are you near the opponent’s end zone or your own? Do you have the high ground for firing archers? Do you control a choke point so that zombies have to funnel into your line of fire? Do you have enough space to build new buildings? These are all examples of territory.
Territory can lead to strategic dominance, increased pressure, fights for resources and advantage. Monopoly and Chess are two examples of games based around owning and leveraging territory. NFL football uses yardage in combination with downs as its primary means of determining whether a team retains possession or not. CityVille constrains available territory and makes it very expensive to do expand.
Unlike other resources, territory is usually fixed to a world. Ownership of it requires that one or more of the players tools (or his own doll) be present in the territory, and when he moves then he loses it. In sim games that kind of scenario is rarely a problem (you don’t lose portions of your city just because you have no buildings there) but the logistics of efficient territory usage matter a great deal. Real time strategy games are a mix of the two.
The requirement of territory as a resource to help mould a game is one of the reasons why ideas like destructible terrain are less interesting than they sound. It’s not true for all cases (Worms and Lemmings, for example), but when a game’s territory can be significantly altered during the course of play, the changes to the game dynamic are often chaotic.
Territory is usually either separated into nodes or are contigous. Nodes are formal separations, like Chess squares or Nine Men's Morris dots, whereas contiguous territories have no formal separation. Territories may also have boundaries (like the line of scrimmage in NFL), defining where tools can legally be placed and other conditions (such as kicking the ball out of bounds).
Like all other resources, territory may have many appearances but usually has one or two standard behaviours that apply throughout the game. Land tends to be land, water tends to be water, dangerous lava sections tend to stay dangerous and sky tends to be sky. Again, this simplification is important for the player and for keeping sense of the game design. Over-complicating the effects of territory is a sure way to create mass confusion in your game.
In Dungeons and Dragons (3rd edition), wizards can create magical items by spending experience points in the manner of a currency. This use of experience points is unusual, but valid and is an example of a meter being used as a currency.
Another example of mixing functions is selling a tool back to a game. CityVille players can demolish buildings to earn back some currency (usually coins). This also has the effect of reducing the population meter.
As a general rule of thumb it’s a good idea to keep different resources in their roles as currencies, meters, tools or territories most of the time. The issue is simply one of complexity leading to opacity.
Resources tend to need to correspond to physical things that players understand in their daily lives. Money should behave like money and health should behave like health. A soldier should be able to hit things and ball should be able to score points.
If money behaves like health and a soldier behaves like a ball, it just gets hard to relate to. The player loses tracking of the physicality of the game, and then actions become arcane. Figuring out how a resource behaves in your game really should be a matter for common sense most of the time, and if it feels weird to you then chances are it will feel weird to players too.
A little weirdness is fine. Even having one deliberately counterintuitive resource as a focus of game design can make for something really interesting. Just make sure that the rest of them are grounded in variables that players can relate to.