by Gareth Gordon
BBC NI political correspondent
Mr McGuinness, here with Mr Robinson, has used the term joint first minister
Martin McGuinness has been called many things but "joint first minister" is a step too far for Peter Robinson.
At least it is when it's an Ulster Unionist doing the calling.
Mr McGuinness recently used the term himself when he arrived for a meeting of the North South Ministerial Council in his home city.
"Joint First Minister of the North" was how he introduced himself - but that was out of Mr Robinson's earshot.
But, more and more the term has been used by other unionists.
The former DUP MEP and now leader of the Traditional Unionist Voice, Jim Allister, has long been calling Mr McGuinness and Mr Robinson "the joint first ministers", probably because he knows it will annoy the DUP.
Now it has entered the Assembly chamber as well through the Ulster Unionists - though it was only when David McClarty, one of the Assembly's deputy speakers, chanced his arm that Mr Robinson blew.
"Republican speak" cried the DUP benches.
Mr Robinson thundered towards the Speaker: "That is a most serious issue when someone who at least should know (though one sometimes might doubt it)... what the position is, that he is prepared to make those kind of remarks is a matter that your office, I think, should be looking at."
His point made Mr McClarty meekly reverted to Mr McGuinness's correct title of deputy first minister when told to do so by the speaker Willie Hay.
Mr Hay went on to warn members: "Let's not play games in this House. We have a first minister and a deputy first minister."
Nobody seems to have told Sinn Féin.
The term joint first minister has appeared on party press releases, and in the party newspaper An Phoblacht, to refer to Mr McGuinness.
Such licence, of course, is probably a natural consequence of the fact that whatever Mr McGuinness's title may say, the office of first and deputy first minister is co-equal - Martin McGuinness has as much power as Peter Robinson.
The term deputy first minister is another example of the constructive ambiguity on which much of the Belfast Agreement is based.
Sinn Féin MLA John O'Dowd blames "a slip up" by the SDLP during the negotiations which lead to the Agreement in 1998.
"We will continue to use deputy first minister, we will continue to use joint first minister," he said.
"Joint first minister, I think, more truly reflects the position which both men hold. Martin and Peter are equals in this office."
DUP MLA Simon Hamilton describes that as "silly nonsense".
"Whatever the legalities, I would invite anybody to go and look at the title above Room 3 in Parliament Buildings that says first minister and go further on down the corridor and you'll see it says deputy first minister over Martin McGuinness's door, " he said.
"And there's a very clear recognition, not just in Northern Ireland but the world over that the first minister is the first minister and the deputy first minister is his deputy."
What is obvious though is that as far as Sinn Féin is concerned there will be no going back to the days when Ian Paisley casually referred to Mr McGuinness as "the deputy or "my deputy".
The name game has changed.
Peter Robinson isn't chuckling.