mistakes do not occur since spelling and pronunciation match each other. When the pronunciation changes, there are two options. The first is spelling reform: The spelling of words is changed to reflect the new pronunciation. In this case, the date of a spelling reform generally indicates the approximate time when the pronunciation changed. The second option is that the spelling remains the same despite the changes in pronunciation. In this case, the spelling system is called conservative or historical since it reflects the pronunciation in an earlier period of the language. It is also called opaque because there is not a simple correspondence between written symbols and spoken sounds: The spelling of words becomes an increasingly unreliable indication of their contemporary pronunciation, and knowing how to pronounce a word provides increasingly insufficient and misleading information on how to spell it. In a language with a historical spelling system, spelling mistakes indicate changes in pronunciation. Writers with incomplete knowledge of the spelling system misspell words, and in general their misspellings reflect the way they pronounce the words. If scribes very often confuse two letters, this implies that the sounds denoted by the two letters are the same, that the sounds have merged. This happened early with . A little later, it happened with , , and . Later still, was confused with the already merged . If scribes omit a letter where it would usually be written, or insert it where it does not belong (hypercorrection), this implies that the sound that the letter represented has been lost in speech. This happened early with word-initial rough breathing () in most forms of Greek. Another example is the occasional omission of the iota subscript of long diphthongs (see above). Spelling mistakes provide limited evidence: they only indicate the pronunciation of the scribe who made the spelling mistake, not the pronunciation of all speakers of the language at the time. Ancient Greek was a language with many regional variants and social registers. Many of the pronunciation changes of Koine Greek probably occurred earlier in some regional pronunciations and sociolects of Attic even in the Classical Age, but the older pronunciations were preserved in more learned speech. Onomatopoeic words Greek literature sometimes contains representations of animal cries in Greek letters. The most often quoted example is , used to render the cry of sheep, and is used as evidence that beta had a voiced