Грамматика русского языка. Часть 7. Имя прилагательное. Глава 11. Прилагательные в предложном падеже, множественном числе. Russian Grammar. Part 7. The Adjective. Unit 11. Adjectives in The Prepositional Plural.

There are only two prepositional plural adjective endings (irrespective of gender).

The two possible endings for prepositional plural adjectives are ых and их (i.e. just like the endings for genitive plural adjectives). These endings are used for all three genders.

Stressed and unstressed adjectives always take the ending ых (unless their stem ends in г, к, х, ж, ч, ш,щ (spelling rule 1).


Examples of the ending ых:

Картины в больших интересных музеях (the pictures are in big interesting museums).
В иностранных школах (in foreign schools).
В важных письмах (in important letters).

The prepositional plural adjective ending is их if:

• The adjective's stem ends in г, к, х, ж, ч, ш,щ 
• The adjective is soft (e.g. cиний, dark blue)
• The adjective is possessive (e.g. мой)

Examples of the ending их:

The adjective's stem ends in г, к, х, ж, ч, ш,щ 
В английских гостиницах (in English hotels).

The adjective is soft:
В утренних газетах (in the morning papers).

The adjective is possessive:
В наших номерах (in our hotel rooms).