Whatever you are making it is inevitable that sooner or later you will have to join widths of fabric together. This is simple until there is a pattern to contend with. All patterned fabrics have a repeated pattern down their lengths. This is called the "pattern repeat" and is measured from a point in one pattern to the same point in the next pattern.
Most patterned fabrics match horizontally across the width. All the cut lengths have to start at the same point in the pattern so that they match at the seams when they are joined. Therefore a cut length has to be a multiple of the pattern repeat.
For curtains, where large patterns are used it is better to have a full pattern at the hemline and a part pattern in the heading where it is less noticeable.
It is advisable to check the fabric for faults before it is cut because suppliers will not exchange cut lengths.