Greek Acrophonic Heraeum One Plethron (U+27AE) is a Unicode letter number from the Ancient Greek Numbers block. It resides on Plane 0 (BMP, Basic Multilingual Plane), has bidirectional class ON (other neutrals), is not mirrored, and has combining class 0 (Not Reordered). In HTML it can be written as 𐅘 or 𐅘.