Yn.

Ya late-developing letter in English. Called ipsilon in German, upsilon in Greek, the English name is of obscure origin. The sound at the beginning of yard, yes, yield, etc. is from Old English words with initial g- as in got and y- as in yet, which were considered the same sound and often transcribed as Ȝ, known as yogh. The system was altered by French scribes, who brought over the continental use of -g- and from the early 1200s used -y- and sometimes -gh- to replace Ȝ. As short for YMCA, etc., by 1915.