An underlying quality, feeling, or meaning that is not directly stated.
“Despite the polite language, there was an undertone of frustration in the complaint letter.”
“The reply had a slightly apologetic undertone.”
In Reading Part 1, a letter can carry a certain 'undertone', such as frustration or gratitude, even when the wording stays polite. Questions about the writer's attitude or tone often rely on recognizing this kind of undertone, so read between the lines.
Synonyms: undercurrent, nuance, hint