It's hard to believe now, but until 2007, the humble pound sign was just a pound sign. Yes, it was also called a hash back then, but who referred to it like that, anyways?The pound sign's transition from insignificant icon to being the cultural identifier for Gen Y (and probably Gen Z, too) is believed to have started with a single tweet . Chris Messina, a former Google developer, probably had no idea that his crazy concept — using pound signs to denote groups — would catch on, turning the world as we know it into a land of #tbts where everything is #beautiful. #truthSince that fateful August day in 2007, which was, incidentally, the same year that the first generation of the iPhone was released, hashtags have taken on a life of their own. We use them as conversational punch lines, as ways to ignite cultural movements such as Black Lives Matter, fuel political activism , and, of course, we now see them affixed to every advertisement and branded message out there.But as the hashtag's use grew exponentially, the understanding of how it's meant to be used has become blurry. Do you include one, five, or 10 with Instagrams? Do you create your own catchy phrase, or use one that's already popular? Can a hashtag be too long or too short?In a quest to answer those questions and more, we compiled the ultimate guide to using hashtags. Click through to learn and tag on.