> For the complete documentation index, see [llms.txt](https://helps.ptengine.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://helps.ptengine.com/en/faq/tag-and-domain/domain-main-vs-sub.md).

# Main Domain and Subdomain Differences

**Main Domain Name**

Also called a first-level domain name, it has only one dot (point) like `"mysite.com"` (or two dots in the case of [co.jp](/en/faq/tag-and-domain/domain-main-vs-sub.md), ne.jp, etc.). The subdomain name comes before this domain name.

**Subdomain Name**

A domain name that comes before the main domain name, with multiple dots in the domain name. For example, `"domain.mysite.com"` is a second-level subdomain name.

**Third-Level Subdomain Name**

Simply add new content before "domain" in cases like `"help.domain.mysite.com"`. N-level subdomain names are added before N-1 level subdomain names.

**Directory**

Similar to a folder that indicates a location on a computer; it is a path string.

`"mysite.com"` is generally considered the root directory of a website, and subdirectories are located under the root directory. Link paths are typically specified by adding "/" at the end of the domain name, like `"www.mysite.com/agent"`. The "/" indicates a subdirectory.

**Note:**

With Ptengine, you can measure all subdomains and subdirectories under the main domain name by simply adding the main domain name.