Geographical adjectives in -isch occur in a more restricted set of environments than its competitor -s. It mainly appears after non-Germanic inhabitant suffixes such as -aat, -iet or -oot(Heynderickx 1994):

Place name	Inhabitant name	Geographical adjective
Azië	Aziaat	Aziatisch
Jemen	Jemeniet	Jemenietisch
Cyprus	Cyprioot	Cypriot

Moreover, the affix -isch appears when the place name ends in -ië and/or the inhabitant name in -iër, as in AustraliëAustralia > Australiër(an) Australian > AustralischAustralian and KaukasusCaucasus > KaukasiërCaucasian > KaukasischCaucasian.

Moreover, the affix appears in RussischRussian, SaksischSaxon and KoerdischKurdish, where -s might have been expected (compare Friesland > Fries(a) Frisian > FriesFrisian but RuslandRussia > Rus(a) Russian > RussischRussian).

Geographical adjectives inflect normally: they take schwa under the usual conditions (see here):

The geographical adjectives refer to a property connected with the place name, roughly: 'from X', 'belonging to X' or 'associated with X'. This means that the inhabitant name that often serves as the base only contributes to the form, not to the meaning of the adjective (Booij 1988). Generally speaking, all geographical adjectives are relational adjectives, which means that they express an absolute property and cannot be used in the comparative and the superlative. If a geographical adjective is used in comparation, with a modifier or with the negative prefix on-, the meaning shifts to qualifying, translatable as 'typical for X'. This process is known as coercion(Pustejovsky 1991), (Pustejovski 1995), (Jackendoff 1997), (Booij 1988), (Booij 2005). Examples are discussed here.

The suffix -isch is cohering and does not influence the stress of the base word; however stress differences between the place name and the adjective are possible due to the influence of the inhabitant suffix (Kaukasus/'kɑukasʏs/Caucasus > Kaukasiër/kɑu'kaziɛr/Caucasian > Kaukasisch/kɑu'kasıs/Caucasian).

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Geographical adjectives ending in -isch serve as input for nominalization with the suffix -e, resulting in female inhabitant names: RussischRussian > RussischeRussian woman. -isch adjectives can be prefixed with on-, which produces a semantic change towards the reading 'untypical for X': een onaziatisch gezichtan un-Asian face.