SAN FRANCISCO — Twitter’s defining attribute has long been its brevity: 140 characters in a post and no more.

That is now set to change. Twitter said on Tuesday that it would test extending the text limit of a post on its service to 280 characters. (In effect, that would double the length of the first two sentences of this paragraph; those sentences, for the record, add up to 140 characters.)

Twitter said the goal was to eliminate what it viewed as constraints that kept people from tweeting more frequently. One significant barrier, according to Twitter’s internal research, has been the stringent limit on character count.

“When people don’t have to cram their thoughts into 140 characters and actually have some to spare, we see more people tweeting,” Twitter said in a blog post.