On the morning of Monday, 6 July 1942, the Frank family moved into their hiding place, a three-story space entered from a landing above the Opekta offices on the Prinsengracht, where some of his most trusted employees would be their helpers. This hiding place became known as the Achterhuis (translated into "Secret Annex" in English editions of the diary). Their apartment was left in a state of disarray to create the impression that they had left suddenly, and Otto left a note that hinted they were going to Switzerland. The need for secrecy forced them to leave behind Anne's cat, Moortje. As Jews were not allowed to use public transport, they walked several kilometres from their home. The door to the Achterhuis was later covered by a bookcase to ensure it remained undiscovered.
Use of the word "concentration" came from the idea of confining people in one place because they belong to a group that is considered undesirable in some way. The term itself originated in 1897 when the "reconcentration camps" were set up in Cuba by General Valeriano Weyler. In the past, the U.S. government had used concentration camps against Native Americans and the British had also used them during the Second Boer War. Between 1904 and 1908, the Schutztruppe of the Imperial German Army operated concentration camps in German South-West Africa (now Namibia) as part of its genocide of the Herero and Namaqua peoples. The Shark Island Concentration Camp in Lüderitz was the largest camp and the one with the harshest conditions.