A stretchy elastic which has a thread omitted from the centre, giving a 'folding' line.  The width given is the flat measure, as this elastic can be used flat or folded. More often it is folded over the edge to bind the fabric.  Sew straight through all layers, ideally using a zig-zag, but its also possible to use a straight stitch, particularly where the elastic is not going to be under great pressure (ie, the top of a bra cup where it has been used to finish the edge. The elastic comes in many different weights and thicknesses.  The type number is in brackets. (Please be aware some other retailers may give the finished width, but we follow the UK manufacturers labelling, which is always to identify the elastic by the flat width before folding).