In English, for example, the word "don't" is made up of 4 letters ("d", "o", "n" and "t"), and one punctuation mark ("'").
However, there seems to me to be no reason for this distinction. Without any of the letters, the word is incorrect, and without the punctuation mark the word is incorrect.
The same applies to "light-hearted", which, without the "-" is not a correctly written word.
I gather that in some languages, for example Spanish, the "n" is a different letter to "ñ", whereas in English that would likely be considered different punctuation (e.g. "naïve" and "naive" are generally considered the same word, and are both correct).
The field is even more muddied by symbols such as "œ", which are not in the alphabet, but also aren't really punctuation!
What causes the distinction between letters (part of the alphabet) and punctuation (other symbols, required as parts of words but not in the alphabet?) - and why aren't "'" and "-" part of the alphabet? Are there written languages which don't make this distinction at all?