In the word "musical", what do you call the letters "al"?
It is a suffix. See explanation.
Some words in English and other languages can be made by adding some letters to other words.
The letters added can be called:
A prefix if they are added in front of the root word.
patient : im - patient; the prefix im is used to form a word with an opposite meaning to the root word
word : fore - word; the prefix fore means before; here a text put before a book - an introduction to a book
A sufix if they are added at the end of the root word.
quick : quick - ly; the sufix ly is used to transform an adjective to an adverb
music : music - al; the sufix al is used to create adjectives from nouns