Lea is the traditional established yarn count for linen.

(The number of lengths each 300 yards long that make 1lb in weight) Metric equivalent:

The number of lengths each 274m long that make .454 of 1kg. 

60s Lea = 36.3 NM approx

The higher the number the finer the linen thread

60's = Finest we supply

3.5's = Thickest we supply