A schulklopfer is the person who calls a Jewish community to prayer in the local synagogue[1]. In modern times, the custom has more or less died out, but it was historically common.
The schulklopfer was usually a beadle, who would perform the task by wandering around the community, knocking on each household's door[1]. In Neustadt, he would knock four times, in the pattern KNOCK - pause - KNOCK KNOCK - pause - KNOCK; Israel Isserlein (a famous rabbi from Neustadt) argued that this pattern was a reference to the Biblical phrase I shall come to thee and bless thee[2] (in gematria, the letters of the first word of this phrase have the values 1, 2, and 1, respectively)[1]. In the Rhine, the custom was to strike merely thrice, in the pattern KNOCK - pause - KNOCK KNOCK[1].
In mediaeval Eastern Europe, the schulklopfer also had the role of individually inviting people to marriage ceremonies (nissuin); the invitations were made to the entire community by the schulklopfer on the morning of the marriage ceremony itself (such ceremonies were usually an evening affair)[3]
The name stems from the Holy Roman Empire (Germany) in the middle ages[1]. Christians in nearby communities sometimes referred to schulklopfers as campanatores (a Latin term meaning bell-strikers) or as Glöckner (German for bell-striker); these were the terms which the Christian equivalents were given[1].