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cùlbackmasculine noun
the
an cùl
after a preposition
a' chùl
of the
a' chùil

air cùl an dorais, behind the door — literally on the back of it.

Every form but the headword, the genitive and the plural is worked out by the spelling rules in the grammar guide, so what you see here is what the guide teaches. Awaiting a fluent speaker’s review.