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horizontal (as opposed to vertical)
side-to-side (as opposed to front-to-back); width; breadth
side (of a box, etc.)
beside; aside; next to
unconnected
横に並んで一緒に座ろう。
Let's sit side by side together.
横を見たら友達がいた。
When I looked to the side, my friend was there.
横に座ってもいいですか。
May I sit beside you?