Abstract: A method of sealing a sheet of woven ribbons of heat sealable plastics material to an article which may be another sheet of said plastics material comprises applying a regular pattern of ultrasonic spot welds between said sheet and said other article. The pattern of ultrasonic spot welds comprises at least two rows of welds offset with respect to one another so that the welds fall on different parallel ribbons of the sheet and in one preferred embodiment welds or adjacent rows are sequentially staggered by the breadth of one ribbon so that welds of each row form repeating diagonal lines, the diagonal lines extending between the top and the bottom row. The method is especially suitable for use with woven oriented plastics material. The invention includes a bag at least one of whose seams is formed by this ultrasonic spot welding method. A particular bag is a gussetted bag in which seams along at least the bottom edges of the gussets are formed by the ultrasonic spot welding technique.