Fractional RHD?

Apologies if this has been asked before, can't find it anywhere, including Urpriest's guide.

If you apply class levels to a creature with fractional HD, like for instance a Rat (1/4 d8 hd), do you pay 1 level for that 1/4 of an HD, or drop it? I know rats can't normally have class levels, it's just for example purposes.

I know D&D usually rounds down but both answers seem odd here.

If it's got less than or 1 HD and you add a class level, I think you replace the 1 HD with the first HD from the class level and build from there, ie drop the fractional HD.

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Originally Posted by Saintheart

If it's got less than or 1 HD and you add a class level, I think you replace the 1 HD with the first HD from the class level and build from there, ie drop the fractional HD.

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I think this is correct. The MM (p.290) says:

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Creatures with 1 or less HD replace their monster levels with their character levels

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This is under the section "humanoids and class levels" but the sentence doesn't specify humanoids. Savage Species says the same for monsters (p.13, p.105). It's 3.0 material but it's the only place which goes into more detail about this than the MM as far as I know. I don't know of anything in 3.5 which contradicts it, so unless someone else does it's still technically valid.