I briefly mentioned hashtags in my previous post as a special application of tagging in Twitter and numerous other Twitter-related sites. In this write-up I cover hashtags in more detail.

Hashtags are special tags defined as # that one can use as part of a 140 character string to define a topic for a tweet. Content-oriented people may think of a hashtag as a virtual “folder”, while journalists may think of it as a “channel” that one can tune to or listen in on.

So I may tag a tweet with #BPM (for a business process management related tweet) or #job for a tweet on a job opening, or both for a job opening in BPM. Note that hashtags are not case-sensitive and can appear anywhere in the tweet. As a specific example, I used the following tweet to announce my previous blog post on metadata and tagging. Note the use of numerous tags (5 of them) along with a shortened link to the post in the tweet:

“my blog post on #metadata and #tagging http://bit.ly/xxQby #web20 #internet #technology”

You seldom will see a tweet with so many hashtags. In fact most people do not regularly utilize hashtags. For one thing it takes time to look up the right hashtags. But for your tweets to have context and reach the right audience it is best that you tag them with the appropriate hashtags.

Special Hashtags

There are some unusual but popular hashtags that any Twitter user should know about. Here are a few of these:

• #FollowFriday or #ff: Used on Fridays to suggest people to follow to your followers and to whoever is tuned in to #followfriday. For example, the tweet “#followfriday @BarackObama” suggests following President Obama, who happens to be very popular with about 1.6 million followers as of the time of this writing.
• #MusicMonday: Monday is music day on Twitter. Used on Mondays to suggest music to your audience.
• #fb: This one is a special tag in the sense that if used at the end of a tweet, it performs a programmatic update to your Facebook account status, if you have installed Selective Twitter Update application on your Facebook account.

Hashtag Directories and Dictionaries

So how do I know what to tag my tweets with, if any? Where can I look them up? How does one define new hashtags? Let’s answer these questions. There are several hashtag sites that provide you with hashtag directories and dictionaries, where you can look up tags and find the relevant ones you need, add a definition to an existing tag, or even define a new tag, look at trends and see the recent tweets on a hashtag.

Below are a few useful hashtag related sites:

• www.hashtags.org includes a hashtag directory. It also tracks the hottest and newest hashtags, usage report on any hashtag and more. For example as I am writing this, #IranElection that people use to share information about the uprising and unrest in Iran following the election fraud, is one of the top tags being used. This site however does not provide definitions of hashtags.
• www.tagalus.org or www.tagal.us is a wiki-based hashtag dictionary. You can look up the definition of a hashtag, and if a definition for a tag is not provided you can actually add a definition for it. You can also define a brand new hashtag and add it to the dictionary. Sometimes people make up a hashtag as they tweet even though it may not be defined. For example while many tweet with #BPM, it didn’t exist in Tagalus till today. I just created it.
• www.tagdef.com is another wiki similar to Tagalus that provides hashtag definitions. However since these are wikis and evolve collaboratively without the control of a central authority, they tend to be incomplete. #BPM happened to be defined in this dictionary, while there are many other tags that are not defined here.

While many hashtags have obvious meaning as they are spelled out, many others are acronyms and shorts for stuff that is not easy to understand and decipher. For example I just noticed that #mw2 is one of the hot trends right now. I had no idea what that was but according to tagalus.com it stands for Modern Warfare 2, a popular video game. Another hot hashtag these days is #gr88 that often appears alongside #IranElection though its meaning is far less obvious. It turns out that it stands for Green Revolution 1388, the current year in the Persian calendar.

Note that you can make up your hashtags as you tweet. The system does not check for spelling, nor does it verify it against a directory. But if you want a bigger audience for your tweets, you should use well-defined hashtags. If you want to use a seemingly new hashtag, it is best to first check the directories for it and its related hashtags. If it is not there, then it is a good idea to define and add it to Tagalus (and tagdef.com) first before using it.

I will be covering more topics on and about Twitter in future posts.