maybe my title needs some work... but I use Wiktionary a lot for declensions, etc and I often see two forms for feminine instrumental adjectives, but my textbook only ever uses, typically, the 'first' one...
For example, https://en.m.wiktionary.org/wiki/%D0%BA%D0%BE%D1%82%D0%BE%D1%80%D1%8B%D0%B9 for который, or for какой.
который -> fem. instr. -> кото́рой, кото́рою какой -> fem. instr. -> како́й, како́ю
The same problem for me exists with singular instrumental for nouns like книга - https://en.m.wiktionary.org/wiki/%D0%BA%D0%BD%D0%B8%D0%B3%D0%B0#Russian
какой -> sing. instr. -> кни́гой, кни́гою
Why are there two forms in these and many other words? What is the reason? When is each used?