T-Ss: Teacher talking to the whole class, such as in presenting a text, explaining grammar, giving instructions for an activity.
T-S: Questions and answers (dialogues) between the teacher and a student, such as in demonstration, checking comprehension.
T-S-S: Teacher initiated dialogues with more than one student, such as in role-playing demonstration and warm-up activities.
S-T: Student initiated conversation between a student and the teacher, such as in asking questions about a rule or an assignment.
S-Ss: One student talking to the whole class, such as in telling a story, reciting a poem.
Ss/Ss: Students working in small groups, such as in practicing conversation, role-playing.
S-S: Two students work in pairs such as practicing a dialogue, carrying out an information gap activity.
SS: Students doing their work individually such as reading, completing an exercise.
Image from Harmer (1998) How to Teach English