Periodicals are classified by content and divided into volumes and issues by time. Volume is a time classification over a period. The "number of periods" here is the number of periods issued in chronological order in 1 year; The "volume" is the chronological number of this publication, which has been accumulated year by year since the year of its publication.

Generally speaking, periodicals have volume numbers and issue numbers, but some periodicals only have issue numbers without volume numbers, which is based on their publication year. Take nature and PNAS as examples. Naturally, it is rolled every two months and once a week. PNAS is published once a year and once a week. When quoting, we usually list the page number or starting page number of the volume and article, but the serial number is generally not given, or if it is given, it will be followed by parentheses. For example, the following example:

Stouffer, A.L., et al., The Function of Proton Channels in SC and the Structural Basis of Inhibition. Nature, 2008,451,596-599.

Stouffer, A.L., et al., The Function of Proton Channels in SC and the Structural Basis of Inhibition. PNAS，2008， 102(05)， 1596-2599。