What is the job of a quoted company chairman? Before the days of corporate governance, the jobs of the chairman and chief executive were often wrapped up in one - they still are in the United States. But a series of UK corporate scandals - including those in the late tycoon Robert Maxwell's empire - discredited the model of supreme boardroom power.
The result was a series of corporate governance codes - with the force of the City establishment behind them - designed to create checks and balances in the boardroom. The chairman's role became largely non-executive and was defined by some as managing the board and keeping the chief executive from indulging in wild excesses.
This left the chief executive with the task of running the actual enterprise.
It is a role which has not been without its troubles. During the worst of the "great panic" of 2007-8, the frailties were exposed. Royal Bank of Scotland chairman Sir Tom McKillop was widely castigated for failing to rein in the excesses of his chief executive, the infamous Sir Fred Goodwin, and then signed off on Goodwin's multi-million pensions deal.
Manchester-born Lloyds TSB chair Sir Victor Blank was required to take responsibility for the disastrous merger with Halifax Bank of Scotland.
One of the key elements of exercising the chairman's role is to be out front in times of crisis. Former City Minister Lord Myners played a starring role in June 2004 when he saw off Sir Philip Green's boarding party at Britain's most iconic retailer, Marks & Spencer.
Roger Carr took much of the pressure off his chief executive Todd Stitzer during this year's heated takeover battle for Cadbury. In media interviews and in the negotiations with Kraft boss Irene Rosenfeld, he was the public face of the company.
At mining giant Anglo-American, chairman Sir John Parker shielded chief executive Cynthia Carroll (one of the few female FTSE bosses) from the wrath of disgruntled investors over poor performance.
In recent crises, however, the chairmen appear to have done much less well.
At Goldman Sachs the weakness of investing all power in one person - executive chairman Lloyd Blankfein - was exposed when the SEC brought its market abuse case over alleged fraud.
Blankfein found himself taking all the fire before Congress and the media. Whereas that might have been fine were he a non-executive chair, it was much more damaging when the same person held both the top posts - it became impossible to separate the operations of the bank from its public image.
Moreover, not all chairmen are ready to stand up and be counted. In the labour dispute at British Airways it has been feisty chief executive Willie Walsh who has been the public face of the carrier.
Chairman Martin Broughton has hardly been seen leaving the impression that somehow Walsh - who is fighting an existential battle for BA - is not carrying his board and shareholders with him. Indeed, Broughton seemed to think it was OK, in the middle of BA's troubles, to take on the equally testing but far more sexy job of sorting out Liverpool FC.
The most obvious absent landlord has, however, been at British Petroleum. Chief executive Tony Hayward has found himself in a fire-storm. At one and the same time he has had to testify to Congress on the Gulf of Mexico spill, deal with a voracious media and run a military-scale damage limitation exercise from Houston as some £30bn has been wiped off the company's value.
Yet the group's virtually unknown chairman, former mobile phone boss Carl-Henric Svanberg, has been unseen. Indeed, many people - even those engaged in business - will have never heard of him. So it is Hayward who has taken the pressure and even recognised publicly that he may have to resign when full consequences of events are known.
In the coming days and weeks we can expect endless speculation about the next chairman at Marks & Spencer. There is no urgency in that Sir Stuart Rose, the public face of M&S, is still in place.
One candidate, Roger Carr, already has ruled himself out amid mixed reviews of his work at Cadbury. Another, Sir Victor Blank, with his investment banking and GUS experience might be seen as ideal. But he could be hampered by shareholder criticism of his role in creating Lloyds Banking Group.
Whoever takes the M&S post needs to recognise that he becomes the public face of the nation's favourite retailer and his every action will be monitored in meticulous detail.
Many chairmen now earn near seven figures salaries. That is simply too much for a figurehead role.
The lessons from BA, BP and other centres of crisis must be learned. The chairman's role is not just to sack the chief executive as some like to think. It is also to provide a shield in times of trouble so that operational duties are not neglected.