Conversation so far:
can two elements form more than one compound
two elements react to form more than one compound, a fixed mass of one element will react with masses of the other element in a ratio of small, whole numbers.
What is an example of two elements forming more than one compound
A fixed mass of carbon, say 100 grams, may react with 133 grams of oxygen to produce CO, or with 266 grams of oxygen to produce CO2.

Question: What is another example
Self-contained rewrite: What is another example of two elements forming more than one compound