Dominant line is established in floral design when we take a look at the entirety of the arrangement. Some designs emphasize vertical line, such as an arrangement of Gladioli in parallel lines.
To facilitate conversation, and to repeat the line of a rectangular table, floral designers are returning to the long and low design. A standard rule to decide if your table arrangement is at the correct height is to place your elbow on the table, make a fist, then put your chin on your fist. If you can see across the table, the design is correct. Flowers may be placed above and below the line of sight.