Let us go then, you and I,
When the evening is spread out against the sky
Like a patient etherized upon a table...
The Love Song of J. Alfred Prufrock by T. S. Eliot
What does against mean in this context? Why is it used instead of across?
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A patient is spread out on a table. That could be the case of patient waiting to undergo surgery. Spread out here is extended on the table.
So the evening is to the sky
as the patient is to the table.
The patient does not take up all the space on the table. Just like the evening does not take up (fill up) the whole sky.
Against means the evening is not taking up the whole sky. It's like a picture on a wall. Or any object against any background. The evening is, in effect, viewed against the backdrop of the sky. Just like the table is the background for the patient if you were viewing the patient on a table from above.
Hope that helps.
Consider the following:
The silhouette of the trees stood out starkly against the evening sky.
Those faces in the crowd were like petals against a wet black bough.
The meaning is of "contrast" and "juxtaposition".
Do you see the Latinate underpinnings of the word "contrast"?
Now, to ask us to explain how evening can occupy the slot occupied by "silhouette" and "petals", that would be to get into literary interpretation, which is off-topic for this site.