Old Nubian script is mostly Coptic, but with three additional characters: two from Meroitic and one (shown above) that they think is a doubled Greek gamma.
In Nubian, the length of the consonants is important, but the length of the vowel is not. In English, the length of a vowel or a consonant is not meaningful: if I say “housecat” and “housecaaaaaaaat”, those both refer to a common pet; if I say “buh-keeper” and “buhk-keeper”, most Canadians probably wouldn’t even hear the difference. However, in some languages, the length of the vowel and the number of consonants is meaningful. In Italian, if you only pronounce one “t” in “sette”, you’ve said “thirst” instead of “seven”. In Japanese,”ba” is “aunt” and “baa” is “grandmother”. In the Nubian script, they distinguished between double-consonants and single in writing, but they did not distinguish long vowels from short ones in writing.
Old Nubian made extensive use of abbreviations, especially abbreviations of sacred names (denoted by a horizontal line over them, as perhaps pioneered by Syriac). They also had symbols that were functionally very close to the modern Latin period and question mark, and sometimes used a double-slash as a verse separator.
The modern form of the language (Nobiin) is tonal, but there are no tone marks in Old Nubian.