“Four tired boots waited at the gate.” Read literally, the sentence describes footwear. In a story about two walkers, however, the boots can stand for the people wearing them. The detail brings their journey and fatigue into view before their names appear.
That use of a part to stand for a whole is synecdoche. The Poetry Foundation’s glossary places it among figures of speech and notes its relationship with metonymy. The label is useful, but the more interesting reading question is what the chosen part makes us notice.
Change the detail and the sentence changes
Try three original versions of the same moment: “Two people arrived”; “Two voices reached the doorway”; “Two hungry mouths waited outside.” They do not feel equivalent. Voices foreground sound and presence. Mouths foreground need and can also make the people seem reduced to that need.
A writer’s choice can therefore be affectionate, practical, comic or dehumanising depending on context. Naming the figure does not settle its effect. Look at who is speaking, who is being described and what the surrounding passage allows them to be.
Metonymy is a broader neighbouring idea involving an associated term. The same glossary’s entry on metonymy helps with that distinction. A part-whole relationship is not identical to every association between two things, although examples and classifications can overlap.
A short exercise for close reading
Choose a paragraph from a book you are reading and underline one concrete detail used to represent something larger. First write the literal meaning. Then write the larger thing it appears to represent. Finally, ask what disappears when the writer uses the smaller detail instead of naming the whole.
If you cannot establish a part-whole relationship, do not force the label. The phrase may be metaphor, metonymy, a literal description or a mixture of effects. An uncertain classification can still lead to a precise observation about the writing.
For your own work, describe a busy station through one part of its activity: wheels, hands, voices or tickets. Then rewrite it using a different part. Compare the mood and the implied point of view.
Synecdoche is most useful when it helps you explain why a sentence feels the way it does. The small part is carrying more than information; it is directing attention toward one version of the whole.