Information: Wikipedia

A Stolperstein literally "stumbling stone", metaphorically a "stumbling block" is a sett-size, 10 by 10 centimetres (3.9 in × 3.9 in) concrete cube bearing a brass plate inscribed with the name and life dates of victims of Nazi extermination or persecution.

The Stolpersteine project, initiated by the German artist Gunter Demnig in 1992, aims to commemorate individuals at exactly the last place of residency—or, sometimes, work—which was freely chosen by the person before he or she fell victim to Nazi terror, euthanasia, eugenics, was deported to a concentration or extermination camp, or escaped persecution by emigration or suicide. As of 29 March 2018, over 67,000 Stolpersteine have been laid in 22 countries, making the Stolpersteine project the world's largest decentralized memorial.

The majority of Stolpersteine commemorate Jewish victims of the Holocaust. Others have been placed for Sinti and Romani people (then also called "gypsies"), homosexuals, the physically or mentally disabled, Jehovah's Witnesses, black people, members of the Communist Party, the Social Democratic Party, and the anti-Nazi Resistance, the Christian opposition (both Protestants and Catholics), and Freemasons, along with International Brigade soldiers in the Spanish Civil War, military deserters, conscientious objectors, escape helpers, capitulators, "habitual criminals", looters, and others charged with treason, military disobedience, or undermining the Nazi military, as well as Allied soldiers.

Salomon Kalker was the son of Jacob Kalker and Hester Walg. He had a shop at Markt 37. Salomon was married to Flora Kalker-Wijsenbeek.

Elisabeth (nickname Betsy) Hakkert, born in 's Grevelduin-capelle on December 2, 1862 as the granddaughter of Philip Hakkert and daughter of Heijman Hakkert and Aaltje van Leeuwen, comes to live in Raamsdonksveer at the age of 58 and a few years later in Geertruidenberg, where she will live there for the rest of her life until she is deported. She remains unmarried.

On April 9, 1943, ten Jewish residents of Geertruidenberg were forced to leave by train to Camp Vught (Konzentrationslager Herzogenbusch). Among them was 81-year-old Betsy Hakkert. She had been in the hospital of Geertruidenberg for months. She had to travel immediately to transit camp Westerbork, the portal of the mass extermination, and was later murdered on 23 April 1943 in Sobibor.

Rika Kalker was married on April 13, 1910 to Rudolph Jacob ten Brink de Bruin, a son of Noach ten Brink de Bruin and Betje Mack. After his death in 1933 she moved in with her brother and sisters.

Salomon Kalker and his sisters Sofia, Rika and Sara lived at the same address. They were all children of Mozes Simon Kalker and Jenette Hartog. One other child of this family survived the war.

A niece Bethrina Kooperberg lived with the Kalker family. The residents of Stationstraat 18 were first deported to Vught. They were eventually killed in Sobibor on the same date.  Bethrina Kooperberg was a sister of Trijntje Kooperberg. She lived with her uncle and aunts. Her parents and three children from the parental family survived the war.