As Michelle Obama's The Look is published, we dissect first-lady fashion through the decades. From "mother of the nation" to Flotus fashionistas, what have their unique styles signalled about themselves, their role and their eras?

"The look, like the woman, was bold, powerful, forward-looking, visionary, and empowering for everyone who dares to believe in the possibility of better tomorrows," writes African-American studies professor Farah Jasmine Griffin in the foreword to The Look, the new book charting Michelle Obama's style evolution. Featuring the voices of longtime stylist Meredith Koop, as well as Obama's makeup artist Carl Ray, hairstylists Yene Damtew and Njeri Radway, it goes behind-the-scenes on the brooches and ball gowns of the White House, through to the double denim of the present day.

It's no small claim for the fashion choices of a woman who was First Lady of the United States (otherwise known as Flotus) from 2009 to 2017, particularly when the task at hand is such a complicated one. But, as Obama recently revealed in an interview with Good Morning America, what she wore wasn't incidental: "I really thought about what I wanted to say with my fashion. I wanted to talk about inclusion, diversity, opening up opportunities, and fashion was one of those tools that allowed me to do that."

She must have known that her style would be picked over and analysed, because what the first lady wears has been the subject of fascination and discussion ever since Martha Washington, wife of George Washington, who was the first ever president of the US from 1789 to 1797.

What the US president's spouse wears – and with a woman still to hold the US presidency, all 45 have been "first ladies" rather than "gentlemen" – isn't just a case of aesthetics. While their husbands have been the officeholders, their dress has at times been highly political, a source of diplomacy abroad, or a show of loyalty at home.

"There is no one goal or message that Flotus is trying to achieve through her dress," historian and author of Dressed for Freedom: The Fashionable Politics of American Feminism Einav Rabinovitch-Fox tells the BBC. "Each first lady is different, and her use of clothing is different." Not addressed in the US Constitution, the role of the first lady has evolved over time from being more domestic – "in the early years of the republic (presidents Washington, Madison), the first lady was basically a hostess and a companion" – to more professional, and political.

In the 20th Century, according to Rabinovitch-Fox, "the role became more important, and thus the image and appearance gained importance too"; she believes this was in large part thanks to Eleanor Roosevelt, who was first lady from 1933 to 1945. She was not only the US's longest serving first lady but a towering political figure in her own right.

As Obama recently told People Magazine: "It was a complicated assignment. The role of first lady is a kind of job, non-job. You know that you're supposed to be inspirational, yet accessible. You should be uniquely yourself, authentic, but representational at the same time."

So what have some of the most well-known first ladies worn during their time in the White House – and how have their outfits reflected not just them but their era?

The first ever first lady, Martha Washington "really shaped the office," says Rabinovitch-Fox. She had to. As Feather Foster, a presidential historian and author of The First Ladies: An Intimate Portrait of the Women Who Shaped America, tells the BBC, she had no precedent for what to wear except from the style lessons of monarchies. But, given the history, "above all, she and her husband George did not want to be perceived as monarchical." Seen no doubt as a patriotic duty, she had, according to Foster, "worn only clothing made in the US from US fabrics since before the Revolutionary War [1776] and this would never change".

Aged 57 when she became first lady she was, says Foster, "focused on dignity and appropriate behaviour". Her gowns "were usually dark or muted in colour. Black, greys, creams and pale blue in summer. But always of high-quality fabrics. Her jewellery was simple. Tasteful rather than opulent".

"Her style was very matronly," says Rabinovitch-Fox, which "fitted the image of the 'mother of the nation'." Plus, "at a time when very few images circulated in the press and other media, Martha's position as a first lady enabled her image to be popularised, and she became one of the most recognisable women in the 18th Century, which also contributed to the popularisation of her style."

The wife of James Madison, the fourth president of the United States from 1809........

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