atop
Word family
Synonym discussion
Sense: preposition/1
All four place one thing on the surface of another, but they differ in register and precision. 'On' is the everyday default and works in any situation: 'a book on the table'. 'On top of' adds the meaning of being at the highest point and sounds natural in speech: 'on top of the fridge'. 'Upon' is formal but doesn't specifically mean the highest point; it often appears in fixed phrases like 'once upon a time'. 'Atop' is the most literary of the four and almost always implies a tall or prominent surface — you'll see it in novels and news headlines describing flags atop towers or eagles atop cliffs, but rarely in casual conversation.
Common collocations
Etymology
Formed in early modern English by joining the small word 'a-' (meaning 'on' or 'in', as also seen in 'aboard', 'aside', 'ashore') with the noun 'top'. The combination became a single word in the 1600s and has stayed mostly in literary and formal writing ever since, while the plainer 'on top of' took over in everyday speech.