hanamaru
Say where something sits
Position words like うえ (on), した (under), and なか (inside) attach to a place with の to say where something is.
JLPTN5
[place] の [position] に
Position nouns are ordinary nouns, so they ride the の you already know: [place]の[position]に means "at the [place]'s [position]." いすの うえに means "on top of the chair." The N5 set covers うえ (on/above), した (under), なか (inside), まえ (in front), うしろ (behind), and となり (next to).