In this documentation you will often hear the phrase trinary-based which is a reference to the unique data structure being utilized in IOTA. Whenever we refer to trinary-based seeds, addresses, hashes etc., it means that the string can only consist of characters in the latin alphabet and 9.

// All possible tryte values
var trytesAlphabet = "9ABCDEFGHIJKLMNOPQRSTUVWXYZ"

Because of the way that Curl works, a single hash always consists of 81-trytes (81 characters) - sometimes it can be 90-trytes (including 9-tryte checksum).

Valid Trytes: VBVEUQY

Invalid Trytes: Vaafd8432