Avagraha (nya log)
The Sanskrit symbol avagraha is transliterated as an apostrophe using the western alphabet, and as a nya character with a reversed tail (nya log) using the Tibetan alphabet.
According to the the rules of sandhi - which govern the harmonious sounding of adjacent words in Sanskrit - when a word ending in either of the vowels "e" or "o" are followed by a word beginning with the short (hrasva) vowel "a", the "a" is elided (i.e. omitted) and replaced with the silent avagraha. Thus e + a = written e' pronounced e; and o + a = written o' pronounced o.
For example, the mantra of emptiness ends with the words śuddho ‘haṃ. The word ‘haṃ is actually the Sanskrit word ahaṃ (meaning the nominative singular pronoun "I") with the short vowel a elided and replaced with the avagraha.