For this exercise I went out and looked for horizontal and vertical lines to photograph.
Here the line is provided by the signposts.
Here they are provided by the metal bars in the concrete.
Here they are in the fence posts.
And finally in the bridge (although it is slightly obscured by the bush in the foreground)
In this photo the vertical line is the post.
Here the lines are the bullrushes.
The vertical line dominates this photo.
And here there are 3 vertical columns.