on
Would there be a difference in meaning here?
The small plates used for resting teacups are known as saucers.
The small plates used for resting teacups on are known as saucers.
The first sentence does not imply that the "resting" of the teacups is necessarily *on* the saucers. Also, the word 'resting' is ambiguous as to whether it is a participle (my impression: "teacups that are resting") vs. a gerund/noun (for [the] resting of the teacups).
The second sentence is definitely clearer in meaning, and has fewer ambiguities.
<The small plates used for resting teacups are known as saucers. >
"Resting teacups" could mean "teacups that are at rest", right?
<<"Resting teacups" could mean "teacups that are at rest", right?>>
right.