For pleated header styles, such as Double Pinch Pleat, Goblet and Inverted Box Pleat, please note:

If the length of the finished curtain is more than 108 inches, the max width without seam of the finished curtain is 56 inches;
if the length of the finished curtain is less than 108 inches, there is no seam for any curtain finished width.
If not, the width should be double-wide with a vertical seam.
For other pleated header styles, such as Triple Pinch Pleat, please note:

If the finished length of the curtain is over 108 inches, the maximum finished curtain width without a seam should be 50 inches.
If the curtain length is less than 108 inches, you can have any finished curtain width without a seam.
However, if the curtain length is exactly 108 inches, or if it falls between 108 inches and the maximum width of 50 inches, then you will need to use a vertical seam to achieve a double-wide width.
For Grommet and Soft Top header styles, please note:

If the finished length of the curtain with grommet header style is greater than 102 inches, then the maximum finished width of a single curtain without a seam is 106 inches.
If the curtain length is less than or equal to 102 inches, then there should be no seam for any finished curtain width.