Fibrous structure having variable numbering yarns

A fiber structure for reinforcing a composite material part, the structure being woven as a single piece by multilayer weaving between a first plurality of layers of yarns extending in a first direction and a second plurality of layers of yarns extending in a second direction,

• wherein at least one of the first and second pluralities of layers of yarns includes at least one layer of yarns of variable weight, each variable-weight yarn including a separable assembly of individual yarns each having a determined weight;
• and
• wherein the fiber structure includes at least one portion of reduced thickness in which the variable-weight yarns present weight that is less than the weight presented by the variable-weight yarns prior to the portion of reduced thickness.