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A chorale (pronounce: "Ko-RAHL") is a hymn which is sung in a Lutheran church by all the people. Chorales developed during the late Renaissance and early Baroque period. Most of them were written at that time.
Martin Luther thought that the congregation (people worshipping in church) should have music to sing that was not too difficult. He thought that the words should be in German instead of Latin so that everybody would understand what they were singing about. Luther wrote the words of many chorales himself. He even composed some of melodies himself, such as Ein' feste Burg (A Mighty Fortress). Sometimes he used the traditional Gregorian Chant melodies from the Catholic worship and added new German words to it, for example Christ lag in Todesbanden (Christ lay in Death's Dark Bonds).
The words of a chorale have a rhyming pattern. Most of the melodies have an AAB shape (the melody of the first part sung twice, then the second part). This shape is called "Bar form" in German.
Johann Sebastian Bach used many chorale tunes, usually adding harmony of his own. He used these in his cantatas. Many of these chorales in four-part harmonies are sung as hymns today in German Protestant churches, and some of the tunes are used in English-speaking countries as well.