What is the White House press pool and why is it in a row with Trump?

When Donald Trump unveiled a helipad at the White House on Monday, his remarks were barely audible in live footage of the event.

This was due to a row over an institution that rarely makes the news – the White House Press pool.

Yet it is at the heart of relations between the US president and the media, and this dispute could have constitutional implications.

Coverage of upcoming presidential engagements, which include the UN General Assembly in New York, could be affected.

It is a group of news organisations that combine resources to follow presidents wherever they are - in the Oval Office, at state dinners, on Air Force One or on holiday.

There are separate pools for TV, photographers and print and radio outlets, as well as a "new media" category.

The idea, conceived decades ago, is that having a pool saves on costs and space and it's not practical for hundreds of media outlets to follow the president around.

Members of the pool share their material - pictures and descriptions of the president's movements - with the wider media.

The pool is always on standby in case an event requires the president to speak to the nation.

This latest row centres on the TV pool which has five members - CNN, ABC, CBS, Fox News and NBC.

It began on Friday when Trump said he was banning three outlets - CNN, Politico and MS NOW - from the White House, accusing them of spreading "fake news" and "fictions and lies" about his team.

A day later, reporters from those organisations found that their entry badges to the White House had been disabled.

On Monday, all the major US TV networks decided to pull out of White House pool duties - in effect boycotting presidential events.

That afternoon came the first test of this post-pool era - the new helipad at the White House.