# Periodical

Type name: [Periodical](https://developers.google.com/workspace/gmail/markup/reference/types/Periodical)

Extends [Series](https://developers.google.com/workspace/gmail/markup/reference/types/Series)

| Name | Type | Description |
|---|---|---|
| issn | [Text](https://developers.google.com/workspace/gmail/markup/reference/types/Text) | The International Standard Serial Number (ISSN) that identifies this periodical. You can repeat this property to (for example) identify different formats of this periodical. |