Solid Digits
One digit, pushed to the limit.
A number of two or more digits where every digit is identical, like 77, 5555 or 999999. Single digits don’t count — “all identical” needs at least two digits to be identical.
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Repdigits are among the rarest structures on the site: each digit length from two to six contributes exactly 9, for 45 total — about 1 in 22,222 numbers. More digits don’t add possibilities: six-digit solids are still just 111111 through 999999.
Every repdigit is automatically a palindrome, and any with 4+ digits is automatically a triple and a quad — so it never appears alone, it arrives trailing a stack of “bonus” hits. By design it is mutually exclusive with Periodic: a repeating unit may not be a single digit, so no number scores both ways.
Casino culture agrees with the combinatorics: the slot-machine jackpot image, 777, is a repdigit — the intuitive “a whole row of the same symbol” rarity is exactly what the counts measure.