upon, up on, on

Most authorities agree that the prepositions on and upon are generally interchangeable. The choice depends on rhythm, emphasis or convention; upon is slightly more formal or poetic.

• I heard your favourite author on the radio last week.
• Paula carefully placed the book on (or upon) the desk.
• When Jacques Cartier first set eyes upon (or on) the St. Lawrence River, he thought he had found the Northwest Passage.
• Gilles set one brick upon (or on) the other, until the makeshift tower tipped over and crashed.
• Many traditional stories begin, “Once upon a time …”

When referring to something in a high place or when using on after a phrasal verb ending in up (e.g., follow up), use the two-word spelling up on (not upon).

• Heather lives high up on Hamilton Mountain.
• Please follow up on the request from Saskatoon.