Usually, earning a Yale degree entails years spent trekking up Science Hill or slaving away in Sterling Memorial Library, culminating in a Commencement ceremony surrounded by one’s peers, professors and family. But some professors received their Yale degree under different circumstances.

Paula Hyman M.A.H. ’86, a Jewish history professor, was sent hers in Israel.

Steven Girvin M.A.H. ’01, a physics professor, received his in a ceremony with University President Richard C. Levin.

Andrew Casson M.A.H. ’00, chair of the math department, said he was never even informed that he had been given one.

But none of these professors are graduates of Yale. All the degrees are honorary masters’ degrees — hence the label M.A.H. They are so inconsequential that most professors do not even list them on their resumes, but without these degrees, they could not teach at the University.

It is a centuries-old Yale tradition, a holdover from more rigid times, and one that several professors said they had not encountered at other schools: to become a full professor here, you must have a Yale degree.

Yale historian and professor emeritus Gaddis Smith said that 100 years ago, most professors at the University were Yale graduates. But times have changed, and today, he said, only a small number of professors have earned degrees from the University. To sidestep the ironclad rule, the University confers honorary master’s degrees on all new professors. This technically gives them a Yale degree, allowing them to teach here.

Many professors have an unconventional story of how they ended up with a Yale degree.

Hyman’s arrived in the mail one day while she was in Israel.

“I got a message that something had arrived,” she said. “It couldn’t be delivered because it didn’t fit in the mailbox.”

She said she chalked up the necessity of the Yale degree to being just another one of the rituals that went with teaching at Yale, describing it as a “vestige of the past.”

“I don’t even remember if I took it back,” Hyman said.

Girvin got his in a more traditional manner. Every year, there is a ceremony in Woodbridge Hall in which Levin makes a speech and confers the honor of a Yale degree upon all new professors who graduated from one of the world’s thousands of other universities. After the degrees are passed out, a reception is held in the Corporation Room. Professors being awarded degrees even wear ceremonial gowns like the ones worn at Commencement.

“[Levin] told us about the first people to get honorary degrees,” Girvin said. “There were relatives of Elihu Yale, and the very first was given to Ben Franklin.”

Levin said the tradition started a little over 100 years ago.

“It’s a nice way of introducing those who are new to our faculty, but who haven’t been educated at Yale, to the traditions and the special features of the institution,” Levin said.

History professor Paul Kennedy M.A.H. ’83 said the rule’s roots can be found in the storied campuses of Oxford and Cambridge. A graduate of those universities would receive a B.A., but could pay one pound and one schilling for a master’s degree five years after graduating.

“This was designed for clergymen of the Church of England who had gone to Oxford or Cambridge, so the vicar of Yarmouth or God knows where could be a vicar with an M.A. rather than a B.A.,” Kennedy said.

Because master’s degrees soon became so readily available, the two universities soon made it required for anyone who taught there to have a master’s degree from the school, Kennedy said.

“But what happens when you appoint someone from the University of London? You give them an honorary master’s degree,” he said.

When Yale was founded, it continued the tradition, even though professors said it is the only school they know of that has done so.

“You’re not considered kosher unless you have a Yale degree,” Lawrence Manley M.A.H. ’90, an English professor, joked. “It’s just a welcome to the family.”

Even so, many professors said they were moved by the receptions they attended.

“It’s a very touching ceremony,” English professor Roberta Frank M.A.H. ’00 said.

Levin said these degrees differ from the honorary doctorate degrees given at commencement, which are more prestigious. For example, President George W. Bush ’68 received an honorary doctorate of law degree from Yale in 2001.

In contrast, Manley said the honorary degree he got was “not something you’d put on your CV.”

Although the rule might seem outdated to some — Smith even called it “a kind of silly practice” — most professors said they see no reason to do away with it.

“It’s kind of an amusing quirk in Yale’s history,” Girvin said.

Kennedy said it was merely a pleasant formality.

“Is it archaic? It is,” he said. “Is it ceremonial? Yes. Is it doing any harm? No.”