Yerach ben yomo (Hebrew: יֶרַח בֶּן יוֹמוֹ, with variant English spellings, also known as Galgal), is a cantillation mark that appears only one time in the entire Torah (Numbers 35:5), and once in the Book of Esther.
In these occurrences, it is followed immediately by a Karne parah, another mark that is found only once in the entire Torah.
The rare trope sound in the Torah appears in Numbers 35:5 on the word alpayim (אלפים, two-thousand), which is followed by an equally exclusive Karne Parah on the word b'amah (באמה, cubit), in the first of four occurrences of this phrase in the verse.
The fourth and final occurrence being on a Mercha-Tipcha, a common set that are recited in a lazy mode as if they are basically being recited without a melody, show the monotony of performing a mitzvah after performing it so many times.
The yerach ben yomo + karne para phrase occurs a total of 16 times according to the Leningrad Codex.