Input (grade 13): NASA defines the term as a cold period between 1550 AD and 1850 AD and notes three particularly cold intervals: one beginning about 1650, another about 1770, and the last in 1850, each separated by intervals of slight warming.
Output (grade 12): It is generally agreed that there were three minima, beginning about 1650, about 1770, and 1850, each separated by slight warming intervals.