Use 'in an undertone' when characters need to speak without being overheard, indicating secrecy or privacy.

Discreet Conversation

'In an undertone' may convey a speaker's disapproval or sarcasm about the discussed subject.

Expressing Disapproval

Characters might speak 'in an undertone' to hide strong emotions like anger or excitement.

In art and design, 'in an undertone' suggests a work's atmosphere is subtle and not overwhelming.

'In an undertone' can be used to highlight a characteristic by lessening surrounding intensity.