Jeff,

Steve explained it pretty well. The "saltire" design itself is not a part of the tartan. When the cloth is woven there are extra bands of white woven in at different positions along the sett of the tartan, in such as way as that when the kilt is pleated, the saltire design appears. These white bands are not included in the portion of the cloth that goes into the making of the kilt.

In other words the cloth is woven specifically with making this kind of kilt in mind. You would not be able to take the same cloth and make drapes, or a dinner jacket, or anything else from it, because you'd have all these random looking white bars all across the tartan.

Only they are not random. They are specifically woven in positions so that when you pleat it to make a kilt the saltire design is created.