Another common use of against is to describe someone or something in contact with, or occasionally supported by another thing. The contact in question does not need to be particularly strong or lengthy. You can, for example, brush past someone briefly in a crowd and still accurately say that you brushed against them.

In this context, against has some similarities with on; and in fact the two are, in some cases, interchangeable. For example, we can lean on or against a wall, and we can put our head on or against a pillow, etc. However, when referring to a position or location that involves someone or something in contact with but not supported by a surface, generally only against can be used:

In the above picture, the gorilla is in contact with the tree, but its weight is being supported by the ground, therefore it would be correct to say both that the gorilla is sitting on the ground, and the gorilla is sitting against the tree.

Further examples:

“I put the umbrella against the door.”

“She moved the television against the back wall.”

“He parked his car against the pavement.”