In the Citation-Sequence system, references are given numbers as they are cited in the text. The numbers are assigned by the order in which the references are cited. For example, if an article by Einstein was the first work cited in your paper, it would be assigned the number 1. A superscript number 1 is placed immediately before the final punctuation of the sentence to indicate the citation.
Einstein provides a lengthy description of this phenomenon1.
The next time the same paper by Einstein is cited in the text, regardless of where in the paper that citation occurs, the number 1 would be used again.
The discussion of this phenomenon still influences scientists today1.
Every time a work is cited that has not already been cited earlier in the paper, it is assigned the next available number. If the author of this paper were to cite an article by Heisenberg next, the Heisenberg work would be assigned the number 2, and a superscript number 2 would be placed at the end of the sentence to indicate the reference.
Heisenberg's feelings on this phenomenon were quite contrary to Einstein's2.
The next work cited that was not already assigned a number would be given the number 3, and so on.
Citing Multiple Sources
In the Citation-Sequence system it is possible to cite multiple works in a single citation. To do this, provide the citation numbers for each work in numerical sequence, separated by commas, in the same superscript. Ranges of numbers, indicated with a hyphen, may also be used, if all works referred to in that range are relevant.
The hypothesized mechanism for this phenomenon was tested by several authorities over the years3,5,6,11-15.