It is further possible to take all the sequences which equal, say, 'V ²' and compare them with all those which equal 'V ³', and so on. In this way we can generalize as to what is added to each numbered symbol to obtain the next higher number for that symbol. In many cases we would find that it is not a unique class, but any one of several classes or sequences, that may be added to a symbol in order to obtain its next higher number. Hence if we are building up a sequence of classes and stop at any point, say when our sequence equals 'V ²', and then ask what we might add to obtain 'V ³', we will often find a great number of possibilities.
- Harris (1951), a pag.278