However where tables have been designed by people for visual convenience, other patterns are common. A graph or scattergram is 2 dimensional in the sense that the X and Y axes represent real values, while the presence or absence of a point specifies a relation between them at that pair of values. A table might be thought of as basically 3 dimensional, in that while the X and Y dimensions can be used to represent two independent values, the number appearing in a cell of the table can specify a third e.g. you could have a table of latitude and longitude values, with height above sea level appearing in the cells. No dimensions e.g. the Macintosh desktop display, or the array of iconic commands in the Hypercard Tools menu or the Clarisworks drawing tools palette, where position has no particular meaning. An entity can be defined using a combination of more than three defining attributes, provided they are enumerable.
Railway timetables use such marks, like footnotes, extensively.
They are used whenever the entities normally have a standard value of some attribute which is therefore not worth representing by a column, but when an entity has a non-standard value then a special mark is put in the extra column or row. Several such attributes can share a column of special marks like this. For example if you know the approximate length, width, and thickness of the timber you want, then both figs. 4 and 5 are quite easy to use and to discover how much it will cost. On the other hand if you have a fixed amount of money and want to discover how big a piece of timber you can buy then fig.4 is quite usable, but fig.