As with most online image boards and forums, 4chan allows you to delete most of your posts at any time. Deleting a post can be a good idea if you change your mind about content you've posted or feel the reaction it's receiving isn't in line with your intended message. In some circumstances however, deleting the post may not be an option.
How to Delete Posts
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To delete a post, check the box in the upper-left corner of the post you want to delete and click the "Delete" button from the list. If you supplied a deletion password when you made the post, enter it into the field. If not, and your browser still retains the automatically generated password, it will be filled in already. If you want to delete the attached picture but keep the comment, select "x File Only" instead and enter the deletion password.
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Posts That Can't Be Deleted
Not all 4chan posts can be deleted. Some threads, like sticky threads generally posted by administrators or moderators, protect reply posts from deletion. Some boards, like the infamous "Random" board, "/b/," also disable post deletion. You may also be prevented from deleting a post if you didn't enter a deletion password, but deleted the cookie that contained the automated password.