Tumblr does not have a native comment system like traditional blogging platforms. Instead, Tumblr posts have "notes," a cumulative count of every time a post has been interacted upon. While the note system is a bit tricky to understand when you are new to Tumblr, eventually following the way people comment and communicate on Tumblr becomes second nature.
The amount of likes and reblogs on a post make up the note count. Every time someone likes a post or reblogs it to their own blog, the original post gets a note. The vast majority of notes are not commentary. However, due to Tumblr's lack of a traditional comment system, many users make commentary by reblogging the post and adding something to the text. As such, you occasionally see long posts of nested text where two or more users commented on the original post and even each other's commentary.
Seeing Commentary in Reblogs
To see what users have said when they reblogged a post, open the post page. If you're on the dashboard, click the upper right corner of the post; the corner will fold over in the style of a ear-marked page. Scroll to the bottom of the page to see the detailed notes on the post. Reblogs are in the format "User2 reblogged this from User1." If User2 added commentary, that note will read: "User2 reblogged this from User1 and added:" with the first few words of the commentary visible. In most themes, the text links the reblogged post.
If the blogger has allowed it in the settings, specific types of posts allow replies. A user can reply to a single post just once and a reply cannot be edited or removed. Replies are visible in the post notes along with likes and reblogs. You cannot reply to your own post. Many users copy replies and respond to them via a text post -- from there, the conversation often expands with reblogged commentary. Not all Tumblr users have allowed replies to posts.
For blogs that want to enable a more traditional comment system, many Tumblr themes support Disqus integration. If a blog uses Disqus comments, the comments and comment form are usually visible below the notes on the post page. Disqus comments on Tumblr behave the same as comments on any other blogging platform.
This article refers to Tumblr settings and user behavior as of January 2014. Features and habits may change with future versions of the network. Also, Tumblr has dozens of themes, and gives users the tools to write their own themes. As such, there's always the chance that a theme may display notes differently than the norm.
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