I was exhausted last night, but couldn't sleep. For some reason, I kept thinking about how many places the 9 cell grid (or scale 9 grid as it is called in some uses) 3x3 grid pops up in our work. In its simplest form, the grid is something like this:

With the middle center often being optional. Or, put into C#, perhaps a structure or class that looks like this:

public class Grid9<T>
{
    public T TopLeft { get; set; }
    public T TopCenter { get; set; }
    public T TopRight { get; set; }

    public T MiddleLeft { get; set; }
    public T MiddleCenter { get; set; }
    public T MiddleRight { get; set; }

    public T BottomLeft { get; set; }
    public T BottomCenter { get; set; }
    public T BottomRight { get; set; }    
}

Where T could be an image or connection point or any number of other things.

In Element Borders

Until HTML5 gets fully adopted, many folks are still going to use some variation on the scale 9 grid to do borders. The image links to one written in jQuery.

Truthfully, even after the adoption of HTML5, scale 9 is going to be very useful. Many Flash and Silverlight designers also use the scale 9 grid; it provides a great way to create complex bitmap assets and use them in a fluid layout. In the case of scale 9, the top and bottom are stretched horizontally, and the left and right are stretched vertically to accommodate the dimensions of the shape.

In Connection Points

There's a fun puzzle game called "World of Goo". In that, you have these little blobs that have to be connected to form sound (ok, not quite sound, but they need to work) structures to allow the goo blobs to get to their destination. The game has fun physics using forces and gravity to make the puzzles challenging.

Each of the little blobs has eight outside points which may be used to connect to it. To the left, you can see one which has seven of the eight points connected by other goo blobs.

BTW, pick up the game. It's addicting like Lemmings, one of my all-time favorite games.

In Grab Handles and Adorners

Most software with any sort of a design surface has settled on using the 8 outside points of the 9 point grid. Some, like the camtasia region selector, use the central point for moving the entire selection box.

Most graphics design software also follows the 9 point approach, as does Window sizing in general.

As Movement Vectors

Analog joysticks typically had four points (top, bottom, left, right) or the full 9. Software that worked with ones that had only four simulated the 9 by checking to see if more than one switch was active (top and middle left would mean top left).

Many games still work on those same eight points for navigation. Most older sprite-based games sometimes create unique sprites for only those eight directions plus a central "at rest" sprite. Modern games typically use hardware to rotate a single shape for overhead-views.

In 2d Transforms

The standard data type for a 2d transform is a 3x3 matrix. Silverlight uses it and many other platforms have used it long before then.

A 3x3 matrix enables you to create transforms that affect both the x and y components of a point. To do 3d transforms, you need a 4x4 matrix.

Others

Of course, there's also uses like the Rubik's cube in the intro, Soduku, Tic-Tac-Toe, and more. It's not as prevalent as, say, a point or vector class, but you may be surprised at where it crops up.