Fabric! Wonderful fabric! Side note - collecting and hoarding fabric is a totally different skill to sewing and dressmaking!
Fabric is a joyous thing, the variety is endless, the creative possibilities boundless, and understanding just a tiny bit about it's creation can help us a lot in our sewing. Fabric is defined by the fibre used, the weight of the fibre, the weave which is applied and any final finishing. Fibres range from plant, animal to man made, such as cotton, silk and rayon. Weaves include plain, twill and satin, among others. Weaving a fibre will then give you a fabric type, for example, cotton twill, cotton lawn, cotton poplin, cotton denim, cotton flannel etc etc.
You could fill pages and pages writing about different types of fabric. Let's focus instead on the general properties of fabric and how we can best use these to give professional results to our sewing projects.
Consider how a fabric is woven on a loom. The loom is first threaded with multiple long threads, these can be several hundred meters long and determine the overall length of the final fabric bolt itself. The physical width of the loom will determine the width of the finished fabric. The fabric is woven within these two constraints for length and width. This is why you can only buy fabric in a small number of widths, as looms only physically come in such widths!
The long threads need to be strong, given their length and how they support the fabric, you would not walk them snapping while weaving. These long threads, the warp, can therefore be of a heavier weight than the horizontal threads, the weft, which are woven across them. The weft passes across the warp from left to right and back again, going under and over the threads, the pattern depending on the type of weave. At the end of each pass, the weft thread loops around the outermost warp thread, this finishes and forms the edge of the fabric, known as the selvedge (self-edge). The selvedge is usually a plain weave, regardless of the fabric weave. You may even find coloured dots on the selvedge, which are check marks for the printer that all the colours have been correctly applied in a print.
Now consider when you come to cut out your project pieces and why where you place them on the fabric is so important. We already know that the straightest, strongest fibres run the full length of the fabric, parallel to the selvedge - this is the grain or grainline of your fabric. In dressmaking, you want to use this strength and straightness in your cutting to ensure garment pieces hang beautifully vertically and do not twist or pull on the body. Using the warp, helps ensure you achieve this. Your trousers will hang beautifully straight, your front coat openings will be straight and not pull, skirts will not twist off to one side, when you are using the fabric's grain.
Running perpendicularly across the fabric, in line with the weft, is the cross grain. You can also use this to align your pattern pieces, however if there is a noticeable difference in weight/strength between the warp and weft threads, the pieces will favour the stronger warp and may pull towards this direction if the weave is not square.
The selvedge itself can sometimes cause problems as, once the fabric has had any final treatments/polishing, the selvedge may be tighter overall than the fabric, pulling it in slightly along the edge. If you see this, it can help to cut the selvedge off the fabric, or even simply snip into it at intervals, to release this tension and ensure your fabric is sitting square.
As the weft threads run horizontally along the fabric, the safest way to get a straight line to work from, is either to rip the fabric - as the rip will follow a weft thread, or to pull a weft thread out from the fabric itself - so showing you a straight line. Comparing this weft line to the selvedge will let you see if your fabric is truly square or if it is at all off grain. If off grain, the fabric can be blocked into shape - sometimes simply pulling the opposite corners can help, otherwise, lots of steam from your iron then pulling into shape, will help you realign your fabric correctly.
With woven fabrics we don't always want to use the grainline when cutting. Consider cutting the fabric at 45 degrees to the selvedge/grain - this is called the bias or true bias. Picture the weave in your head, how the threads move over and under each other, see them as lots of 'square boxes' made by the fibres going over and under. If you were to push or pull at these 'boxes' either horizontally or vertically, there would be little movement as they are held firmly in their 'square' woven structure. Now think about pulling or pushing these 'boxes' at opposite corners, diagonally now there is movement, the 'boxes' can collapse on themselves, moving from a 'square' to a 'diamond' shape, this is the stretch you feel when working with the bias. Try it with a piece of woven fabric - pull it horizontally, pull it vertically - there will be little movement or stretch in it. Then try pulling diagonally across the weave, the fabric will move and stretch, this is the weave disforming - the 'boxes' collapsing! A fabulous property of woven fabric which can be used to great effect in dressmaking with various draping techniques.
Fabric is a fascinating subject. Understanding the warp and weft and how fabric is woven, helps us to understand how fabric behaves and and how we can best use these properties when we cut out our project pieces.