Character development in Final Fantasy X takes a different path to the usual system of levelling up, instead relying on a device called the Sphere Grid. The grid is a complex maze of nodes and pathways which must be navigated in order to gain new strength and abilities.
Moving around
As characters gain AP from winning battles, they also gain Sphere Levels as with the standard system. The difference is that these are not permanent, and must be spent in order to move around the grid. This can be done at any time outside of battle, at a cost of one Sphere Level per node moved. It is possible to backtrack over a previously-covered path at the rate of four nodes per level.
Everyone moves independently, spending their own levels to cover what is essentially their own version of the grid; the cheap backtracking costs are not counted for routes others have taken. Characters can happily move through each other, and even come to rest on the same node if need be.
There are some rare items which allow a character to jump around the grid without spending any sphere levels.
Activating nodes
The key to development is in actually using the nodes as you pass by. A character can activate the node on which they stand, or any node one step away regardless of whether or not they ever visit it. Activation is accomplished through Sphere items, with different nodes requiring different spheres. Once activated, the character immediately gains its properties, either boosting their attributes or acquiring a new skill.
Naturally, a character may only activate each node once, after which they should move on in search of more. The same node, however, may be activated by others too as they pass by. The small ring of lights surrounding each indicates those which have. In the example on the right, the little blue diamond shows that Tidus, and no one else, knows this ability.
Going further
Some nodes take the form of locks, preventing anyone from passing through. This may be to restrict access to powerful areas, or to prevent characters from "meeting" too early and losing their specialised status. Lulu, for example, is confined to the black magic areas by a number of strategic lock nodes. They can be "activated" in the same way as any other node, by using an appropriate Key Sphere. This permanently removes the lock for everyone, turning it into a plain empty node.
It is even possible to manipulate the layout of the Sphere Grid by using special node-changing spheres, filling empty spaces with attributes and even clearing unnecessary ones to make room for more. It is possible to gain tremendously inappropriate strength this way, but it will take a very long time.
The grids themselves
Plan your journey in advance with a full map of the grid, painstakingly recreated in orthogonal form. It is over 1,000 pixels wide, and nearly as tall, so be warned.
Alternatively, there's a tabular version which may prove easier to read, but may also cause some browsers difficulties. It's big.
Grids, plural?
Final Fantasy X International and PAL versions feature an additional Sphere Grid, the Expert Grid. This one contains a few abilities not present in the standard grid, and is much more free-form. This allows characters to wander almost wherever they choose, without the restrictions imposed by the other. As such, it allows for a variety of styles, such as a party made entirely of black mages or one in which Yuna is the prize fighter. It also means a more tactical approach is required, to ensure that the party is prepared for any eventuality.
A map of the Expert Grid is forthcoming, but it is proving much more difficult to render. Sorry.
