Dictionary of Spoken Russian/Russian-English/Section 7

§7. NOUNS WITH ПОЛ-

When nouns are compounded with пол- "half" they have the endings of the gs in the ns and as form; in all other case forms they have the usual endings, but пол- then optionally appears as полу-. Thus beside дюжина "dozen," as дюжину, gs дюжины, is дюжиной, there is полдюжины "half dozen," as полдюжины , gs полдюжины or полудюжины, is полдюжиной or полудюжиной, and so on.

In the nouns полдень "noon" and полночь "midnight," пол- has another meaning, and the inflection, though irregular, is different.

Nouns with the prefix in the form полу- do not have the above peculiarity; thus, полуостров "peninsula" inflects just like остров "island."