I showed how to spot the different ways of hemming a lining and explained why it would be better to do by hand or by the special felling machine, since allowances inside are felled in place. But what does that really mean?
Have you ever got your trousers back from the cleaner and there was a phantom crease near the new crease in your trouser? A result of wear, humidity and the cleaning process, the trouser loses some of its crease. If the presser at the dry cleaner is not extremely careful, he will not make his new crease in exactly the same place as the old one. The same can apply to hems and vents- during construction they are usually basted in place so when they are creased, they are creased exactly where we want them to be. If, as a result of wear, humidity or cleaning the hems fall or the vent puffs, it can be difficult to get the crease back in the same place as the old one. One of the goals of tailored clothing is for the garment to maintain its original shape for its useful life; in order to prevent hems from sagging and vents puffing and lining and facings falling out of shape, we tack as much as possible in place, which is permanent. Looking at the photo of the back panel (which has side vents) I am turning up the vent underlay to show how this underlay has been invisibly felled in place using a zig-zag-shaped stitch. Along the bottom, you can see (though not all that clearly) that the hem has been cross-stitched to the cotton interlining, which has, in turn, been stitched to the garment fabric using the same type of stitch as the vent underlay. This can also be done using special blind-stitching machines. This felling prevents the lining from being sewn in using a conventional lock-stitch machine, which is why it must be felled using the felling machine or by hand, as illustrated in the post on hems. So when you see a hem sewn by lockstitch machine (the bright blue lining in that post), you are seeing the manner of closing the lining but you also know for certain that the allowances have not been felled in place, only tacked at the seams, unless glue was used to hold things up, which itself can wear out after wearing and cleaning; it is also almost guaranteed that the facing has been glued in place rather than felled with a blindstitch.