Numbers are classified into different categories. Usually people learn the traditional beginning numbers of the list which are one through ten. After that, people learn the numbers eleven through twenty. Once those numbers are learned, they will detect a pattern in the numbers. For instance, twenty is the next number after nineteen. Then each of the next nine numbers will be joined after the first number in that series. An example would be twenty-one, twenty-two, twenty-three and so forth. Also, numbers are placed in other categories as well. Odd and even numbers are certain groupings of numbers based on the last digit in the number. Beginning with one, every other number is an odd number. Beginning with two, every other number is an even number.