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Letters Rogatory
A Letter Rogatory is a formal request from a court in one country to "the appropriate judicial authorities" in another country requesting service of process.  Although statutory authority generally refers to the instrument as a "Letter Rogatory", the terms "Letter Rogatory" and "letter of request" are synonymous in actual practice.
Letters Rogatory can be used in civil and criminal matters, and have been used in administrative matters.  The execution of a request for judicial assistance by the foreign court is based on comity between nations, absent a specific treaty obligation such as the Hague Service Convention.  If there is no Consular Convention in force between the United States and the foreign country, Letters Rogatory are received by foreign authorities on the basis of comity.
The State Department offers further information on Letters Rogatory here.