Horizontal and vertical lines usually have different visual associations than diagonals.

• Vertical lines often suggest strength, power, growth, or upward reach. Examples include columns, tree trunks, and tall buildings.
• Horizontal lines often suggest calm, rest, balance, and stability. Horizons, oceans, and flat landscapes are classic examples.

They can also do more than set mood:

• Division/separation: A horizontal horizon divides land and sky; strong verticals can separate spaces or subjects.
• Connection/linking: A line can visually connect one part of the image to another, depending on how it is placed.
• Framing: Vertical and horizontal elements such as door frames, windows, walls, and ceilings can frame a subject while leaving more of the image area for the subject itself.

So they are not simply the “opposite” of diagonals. Diagonals tend to feel more dynamic, while horizontals and verticals more often create structure, order, and stability, with specific connotations depending on context.