Prayer and Religious Articles: Prayer and Blessings: Changes in the Prayers:
How do changes in the prayers come about?
The post-Talmudic evolution of the Siddur (and the Torah Readings)
resulted almost entirely from developments and divergences in custom,
not from Rabbinic decisions or "decrees". Orthodox Rabbis occasionally
try to add things to the Siddur -- for example, elegies for
the victims of the Holocaust or prayers for the State of Israel.
If such things become widely accepted, they appear in new editions
of the Siddur and become part of the standard liturgy.
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