Horizontal lines are straight lines that run from left to right, parallel to the horizon. They are often used in art and design to create a sense of stability and calmness. In geometry, a horizontal line has a constant y-coordinate, meaning it does not rise or fall as it moves along the x-axis. In everyday life, horizontal lines can be found in various places, such as the edges of tables, the horizon in a landscape, or the lines on a graph. They play a crucial role in creating balance and structure in visual compositions, making them essential in fields like architecture and graphic design.