The regular, unaccented Greek characters as well as the characters with tonos and diaeresis can be found in the Greek and Coptic block.
Greek Extended was encoded in version 1.1 of the Unicode Standard.
As an alternative to Greek Extended, combining characters can be used to represent the tones and breath marks of polytonic Greek.
Decomposition of U+1F71 ά GREEK SMALL LETTER ALPHA WITH OXIA, for example, yields U+03B1 α GREEK SMALL LETTER ALPHA followed by a U+0301 ◌́ COMBINING ACUTE ACCENT, while composition yields the same letter with tonos, U+03AC ά GREEK SMALL LETTER ALPHA WITH TONOS, from the Greek and Coptic block.
The following Unicode-related documents record the purpose and process of defining specific characters in the Greek Extended block: