As a general tendency, Greek will attempt to avoid beginning a syllable with a vowel or placing two vowel sounds adjacent to each other without a consonant or a glide between them. So prepositions that end in a vowel will typically drop that final vowel if the following word also begins with a vowel. The dropped letter, or elision, is marked with an apostrophe. So, we find κατὰ γραφάς—notice γραφάς starts with a consonant, so [in] κατὰ, the final α can remain. Also I should note with prepositions,