Whereas most other social media sites tend to have very rigid methods of commenting, on Tumblr there are three ways to “comment” on a post: commenting in tags, replying, and when you reblog. Each method has different social connotations which are important to understand to making sure you’re polite, as well as being able to find what people say about your posts, too.
In short:
Also known as talking in the tags, this is the practise of commenting on a reblogged post in the tags. It’s a very unique facet of Tumblr interaction which may be very foreign to users coming in from other sites.
Talking in the tags allows you to comment on a reblog in a way that allows you to share your thoughts on a post with your followers, without adding it as a permanent fixture of the post for people reblogging from you (tags do not get copied when you reblog something).
Some fine art connoisseurs enjoying a good piece of art.
I cannot stress how common this is—Tumblr’s staff have added a feature that lets you look at reblogs of a post who have added tags specifically for this purpose and it’s even mentioned in the official help centre. If you’re going to be on Tumblr for the long haul, expect to use this method of commenting the most, so get used to doing it!
When in doubt, if you’re not sure how to comment on a post, put it in the tags!
<aside> 💬 The rough Twitter equivalent would be retweeting something, then composing a new tweet after starting with “LRT:” or similar, where you want to give commentary to your followers but don’t want to clog up the replies of the original tweet.
</aside>
See the page on using tags on Tumblr to learn about the other ways tags are used.