This is the 11111th most frequent Russian word.
горизонтально
Horizontally.
Describing the position or manner of how the book is placed requiring the use of an adverb.
Книга лежит горизонтально на столе.
The book lies horizontally on the table.
Describing the manner in which the drawing action was performed, emphasizing the direction and alignment of the lines.
Он рисовал линии горизонтально на бумаге.
He drew lines horizontally on paper.
Explaining how the painting was hung emphasizing its orientation being parallel to the ground.