[jacobf]

So, is it only attached to nouns that have counters? Or is the つ itself a counter? That is what is confusing me.

The book gives the example of two people, and it is 二人 (ふたり) and then the second (date); (for) two days, which is 二日 (ふつか)

I think it is called okurigana, the concept itself doesn't confuse me, but I don't understand why it is attached to these kanji.