The book, with its red-checked cover, was not written with an eye toward publication. Her private thoughts and observations were for her eyes only, very much "The Diary of a Young Girl," as its title suggests.
Now researchers in the Netherlands have discovered in the diary a secret hidden for decades, obscured from prying eyes by Anne, who meticulously pasted brown paper over the pages.
On two pages, Anne, who was 13 years old at the time, penned four "dirty" jokes and more than 33 lines explaining sex, contraception and prostitution, the Anne Frank House announced Tuesday. The entry, dated September 28, 1942, also included five crossed out phrases, the museum said.
The recently publicized pages of hidden text serve to highlight "Anne the girl," and her "inquisitive" and "precocious" personality, the Anne Frank House said.
"Anyone who reads the passages that have now been discovered will be unable to suppress a smile," Frank van Vree, director of the Netherlands Institute for War, Holocaust and Genocide Studies, said in a statement. The 'dirty' jokes are classics among growing children. They make it clear that Anne, with all her gifts, was above all also an ordinary girl."
The passages were revealed by image-processing technology. Using photos of the pages taken in 2016 as part of a regular check of the diary, software helped decipher the words, the statement said.
In an effort to save pages of her precious diary that had been "spoiled," Anne decided to use some of the space to jot down risque jokes she may have heard from her father or on the radio, the Anne Frank House said.
"A man had a very ugly wife and he didn't want to have relations with her," she wrote, as translated from Dutch by the Associated Press. "One evening he came home and then he saw his friend in bed with his wife, then the man said: 'He gets to and I have to!!!'"
She also included this suggestive quip: "Do you know why the German Wehrmacht girls are in Holland? As mattresses for the soldiers." (The Wehrmacht was the German term for the country's armed forces.)
Following the jokes, Frank delved into sex education, pretending in the entry that she was teaching someone else, the museum said.
While attempting to explain these "sexual matters," Frank used highbrow phrases such as "rhythmical movements" to describe sex, and "internal medicament," to talk about contraception, the New York Times reported.
Being a teenage girl, she made sure to include a section about periods too, saying menstruation is "a sign that she is ripe to have relations with a man but one doesn't do that of course before one is married," according to the AP.
On prostitution, Frank wrote, "All men, if they are normal, go with women, women like that accost them on the street and then they go together." She added that "In Paris they have big houses for that. Papa has been there."
It is unclear why Frank chose to cover her musings with the sticky brown paper, but researchers say she may have been trying to hide the content from her father or anyone else living in the cramped attic quarters.
Despite being shrouded in mystery for more than 70 years, the uncovered pages "do not alter our image of Anne," the Anne Frank House said. The writings were not unusual for the teen, who had other diary entries about sex and "regularly" recorded dirty jokes, the museum said.
The pages are significant because they show Frank's first attempts to develop her literary voice, the AP reported.
Ronald Leopold, executive director of the Anne Frank House, told the AP the entry is an early example of how Frank "creates a fictional situation that makes it easier for her to address the sensitive topics that she writes about."
"Given the great public and academic interest we have decided . . . to publish these texts and share them with the world," Leopold said in a statement from the museum. "They bring us even closer to the girl and the writer Anne Frank."
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