question of partition in double integral

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in the definition of double integral,what the first to do is to divide region into small subrectangles . my question is :do these partitions have to be rectangles? for example ,when dealing with polar coordinates,region is divided into many small polar rectangles which are not rectangles.so can I use other shape to divide a region ,for exmple:parallelgram or triangle ,Are these also called double integral? if these are also double integral why does the definition of double integral only use subrectangle for partition