This is primarily a G-Rated blog, although some material is PG-13, some is rated R, and never anything NC-17. We’ve shown one female nipple but it was made of terry cloth and therefore hard to determine the appropriate audience.
The NY Times reports that the MPAA, owner of certification trademarks for the ratings system I just used, has send a demand letter to authors of fan fiction who have been using the system to self-rate their content (so that no one looking at, for example, some Harry Potter fan fiction, inadvertently encounters some age-inappropriate tricks).
Other rating systems exist of course – The Entertainment Software Rating Board has the CEETMA system. Television has a strange inconsistently applied system that requires deciphering. There is also an Internet Content Rating Association.