white elastic 15mm wide, fold over line on one edge. Sold by 3m length

white elastic 15mm wide, fold over line on one edge. Sold by 2m length
 
Can be used as standard 15mm elastic, but has further option to use this as fold over elastic to gather and neaten edges. This type has the benefit of not having to cut back the elastic to reduce bulk (as you may need to with traditional foldovers with a central fold line). If you haven't used elastic in this way, have a look online for FOE or fold over elastic videos for lots of creative ideas.