Name: Laurie Who is asking: Parent Level: Elementary Question: What is a group of three numbers called in a large number? My son has this question on his 4th grade worksheet. I've taught middle school math for 7 years and nver heard of it. Hi Laurie, I INTERPRET the question as follows: for numbers over 9999, the digits are grouped into sets of three (and separated by commas or spaces) - this is sometimes referred to as periodicity, or period, or period value (although I only know this because I looked the term up in a math dictionary from 1933 just to make sure I remembered correctly) for example, in the number 123,456,789 789 is the 1st period and is units (i.e. seven hundred and eighty nine UNITS) 456 is the 2nd period and is thousands (i.e. four hundred and fifty six THOUSANDS) 123 is the 3rd period and is millions (i.e. one hundred and twenty three MILLIONS) Period is like place value except for place value we look at individual digits (e.g., the 5 in the above number is in the 10 thousands column and represents 5 ten thousands or 50 thousand), whereas period looks at 3 digits in a group. Period is the concept needed to properly name numbers over 9999. I hope this is the interpretation your teacher was looking for. Paul Go to Math Central