654321

654321 is a six-digit decimal number consisting of the digits 6, 5, 4, 3, 2 and 1 in strictly decreasing order. It is the largest six-digit number that uses each of the digits 1 through 6 exactly once, when arranged in descending order.

Numerical properties: The number is odd, since its last digit is 1. The sum of its digits is 21, which is divisible by 3, so 654321 is divisible by 3, and 654321 equals 3 times 218107. Therefore the number is composite. It is not divisible by 2, 5, or 9. Its digital root is 3.

In usage, 654321 is commonly cited in puzzles, teaching materials, and programming examples as a representative decreasing-digit permutation and as a convenient six-digit example. It is also sometimes used as a test PIN or sample code, though such practice is discouraged for security reasons due to its familiarity.