There is no such thing as an “interim senator.”

The Senate Historian’s office doesn’t keep statistics on that designation, and might even gently chide you for uttering those exact words because once a person is a senator, he or she is a senator for life. It doesn’t matter if he only served four days — like Louis C. Wyman, R-N.H., from Dec. 31, 1974 to Jan. 3, 1975 — or more than 51 years, like Robert C. Byrd, D-W.Va. A man can have the word “senator” etched for eternity onto his gravestone either way (see: Burris, Roland W., D-Ill.; 2009 to 2010).

In the history of the Senate, 125 senators have served for six months or less, including Sen. William “Mo” Cowan, D-Mass., who wrapped up the final days of a five-month stint last week. And 193 members have been appointed to the chamber since 1913, when the Constitution was amended so that voters directly elected senators. Of those, 62 senators did not seek re-election. Sen. Jeff Chiesa, R-N.J., sworn in last month, will fit into all three categories after October, when a special election is held to replace the late Frank R. Lautenberg, D-N.J.

“Not one person was given a chance to vote for or against me, but I have gone about my work every day as if they had,” Cowan said in his final floor speech on June 26, following Rep. Edward J. Markey’s special-election victory to fill Secretary of State John Kerry’s old seat. “I came to this body beholden to Massachusetts, her residents and the country only, and leave confident that I have stayed true to that honor.”

Of the senators currently serving in the 113th Congress, Lisa Murkowski, R-Alaska, Robert Menendez, D-N.J., John Barrasso, R-Wyo., Roger Wicker, R-Miss., Michael Bennet, D-Colo., Kirsten Gillibrand, D-N.Y., and Dean Heller, R-Nev., all were appointed before winning their own elections outright.

There’s also been a wide range of experience for the short timers.

Sen. Ted Kaufman, D-Del., for example, filled in for his longtime boss, Joseph R. Biden Jr., after decades as a top aide. Kaufman served alongside Burris, who replaced President Barack Obama and had no Washington experience at all, and Paul Kirk, who served for 134 days following the death of his former boss, Edward M. Kennedy.

“I was in a very different situation than most interim [senators] because I had worked in the Senate for 22 years, 19 as chief of staff for Sen. Biden, and so I thought a lot about the Congress … [and] had more standing as the average interim,” Kaufman said in an interview, citing a bipartisan breakfast group he attended as a senator with former Senate Parliamentarian Robert Dove to illustrate his point.

“We got to talking about reconciliation in the budget act in 1974, and the only people in the Senate then were Bob Dove and me,” Kaufman said. “In general, it’s hard for someone who comes in and doesn’t have any experience because they don’t really have much to add.”

Kaufman conceded, however, that the amount of access available to members themselves, via requests to agencies or even weekly caucus lunches, was so much more than he realized as a staffer. Kaufman said his experience as a senator has made him a better professor at Duke University’s law school, where he teaches a class on Congress.

Senators who pass through Washington too quickly to become “of it,” as insiders like to say, sometimes seem lighter as they bound through the Capitol. Though he rebuffed such a characterization at first, Kaufman quickly corrected himself.

“Roland Burris. He’s absolutely loved it. He loved presiding. He loved every little piece of it,” Kaufman said. “And Carte Goodwin. … It wasn’t like smiling happy going to a football game, but it’s mostly the impression that it’s so cool to be here.”

Goodwin, who served for 123 days in 2010, is the youngest former senator at age 39, and with deep connections to West Virginia politics, is one of the few former short timers who could one day return.

On a random weekday morning in June, CQ Roll Call got Burris on the phone. He had been watching C-SPAN 2 that day, when senators were debating the “nuclear option” to blow up the procedural rules of the body to try to reduce the number of procedural filibusters to block nominees. Burris said he was watching Sen. Roy Blunt, R-Mo., make the case that “there’s another body and if you just want majority rules, then you go to the House,” in the former Illinois senator’s words.

Burris said he didn’t know much about Senate procedure before getting to Washington; he said he spent nights reading up on “what putting bills on the tree means” and learning the location of meetings.

He said people outside Washington don’t really understand the processes that take place inside it.

“One reason why I know that is because I started a school out here in Chicago. …  I teach at my own school for a grand total of $50,” he said.

Regardless of experience going into their short tenures, each member recalled colleagues of both parties fondly. Burris said he became close with Republicans Orrin G. Hatch of Utah and Pat Roberts of Kansas and that they used to tease each other.

“They were trying to get me on the Republican side because they thought the Democrats were messing with me,” Burris said with a laugh.

Cowan spent a large chunk of his farewell speech thanking his colleagues and highlighting their human sides.

“Sen. Rand Paul and I have recounted our days at Duke and affection for college basketball,” Cowan recounted. “On a bipartisan congressional delegation to the Middle East, I traded life stories and perspectives with Sens. [Amy] Klobuchar and [John] Hoeven and discussed the comedic genius of Will Ferrell with Sens. Gillibrand and [Lindsey] Graham.”

Kaufman talked about bringing his granddaughter to the senators-and-family-only dinner before the State of the Union and said that members of both parties had told her how great her grandfather is.

“If you spend time around any senator, there’s a reason why that senator is there, why their state sent them to the Senate,” Kaufman said.