Also called pronominal adjectives, the Naughty Nine are 9 adjectives that look mostly like first and second declension adjectives, except that their genitive singular ending in all genders is "-īus" and their dative singular ending in all genders is "-ī".
They are best remembered with the acronym UNUS NAUTA.
Ullus, -a, -um (any)
Nullus, -a, -um (no, not any)
Unus, -a, -um (one)
Solus, -a, -um (only, alone)
Neuter, -tra, -trum (neither (of two))
Alter, -tera, -terum (the other (of two))
Uter, -tra, -trum (either (of two))
Totus, -a, -um (whole)
Alius, -a, -um (other)
Notice how these are grouped together.
At the top, "ullus" and "nullus" are grouped together because not only do they rhyme, but they both relate to the word "any".
The next two, "unus" and "solus", are grouped together because they typically will only be used to describe a singular noun.
The next three all end in "ter" and all refer to one of two.
As for the last two, they're just there because they feel like it.