Image copyright EPA Image caption Trump executive orders include building a wall with Mexico

Barack Obama used them sparingly; Donald Trump issued at least four in his first week in office: executive orders are a key tool for any US president wanting to make their mark on government policy.

Republicans accused Mr Obama of overreach when he signed orders on healthcare and gay rights, but they have praised Mr Trump for using the same tactic to roll back Obamacare.

What is an executive order?

It is a written order issued by the president to the federal government which does not require congressional approval.

Orders range from dramatic reverses of policy, like Mr Trump's approval for the construction of two bitterly contested oil pipelines, to ordinary business, such as Mr Obama's instructions on half-day closing for government departments on Christmas Eve 2015.

Authority for issuing the orders is rooted in Article II of the US constitution, which states: "The executive power shall be vested in a president of the United States of America."

Why do presidents issue them?

Image copyright Getty Images Image caption Roosevelt issued several thousand orders

Sometimes the orders are made during wartime or to avert a domestic crisis. In February 1942, President Franklin D Roosevelt signed an executive order that led to the creation of detention centres for about 120,000 Japanese Americans.

In 1952 President Harry S Truman issued an order that put the steel industry under the control of the government in an attempt to avoid a strike.

Mr Obama issued his most controversial executive orders in an attempt to work around gridlock in Congress caused by intransigent Republican opposition.

"If they're not going to do anything, we'll do what we can on our own," he told a crowd in Kansas City in July 2014. "That's when we act - when your Congress won't."

Who can overturn them?

An executive order has to work within the confines of the law, with, in theory, each one "reviewed by the Office of Legal Counsel for form and legality". This does not always happen.

If an order is deemed to stray outside the boundaries of what is acceptable, it can be subject to a legal review.

Congress can also pass a law to override the executive order, but a president still has a veto over that law, the Constitution Center explained.

Why are they politically sensitive?

Image copyright AP Image caption Republicans in the House, led by Speaker John Boehner, at microphone, blocked virtually all of Obama's legislative agenda, leading him to take executive action on his own

Executive orders are controversial because they bypass approval from Congress, allowing the president to act on his own.

Republicans successfully sued Mr Obama over part of his 2010 healthcare changes, arguing that he had exceeded his constitutional authority when he unilaterally delayed an insurance coverage deadline.

A president may also issue an order when members of Congress act too slowly or when the president feels he needs to flesh out details of a new law.

How frequently are executive orders issued?

Image copyright Getty Images Image caption Grover Cleveland issued 28 executive orders per year during his first term

Roosevelt issued 3,721 orders during his 12 years in office, while Mr Obama signed just 279. George W Bush issued 291 over his eight years in power.

Mr Obama's was a modest total by modern standards. His average of 35 orders per year is the lowest tally since Grover Cleveland, who was president from 1885-89, and then 1893-97 - and averaged 32 orders a year.