Flat lining is when you layer 2 fabrics together and treat them as one piece.
Why do this? Usually you do it to stiffen or give more structure to a flimsier fabric. For example, say you have a gauzy material on your outfit. We would call this the “fashion fabric” – the pretty fabric the world will see. Gauzy material is very fragile though and might not hold up well to a lot of strain, so you would flatline the gauzy materialy with something sturdier to help strengthen it and give it more structure.
Some flat lining materials that I’ve used in the past: denim, twill, canvas, and bridal satin.
So here’s a fast example of how to do flat lining…
I am making a 1910s era corset and I’ve picked out my fashion fabric. It’s a cotton though and certainly won’t hold up to the strain when I start lacing up the corset, so I’m going to need to flat line it.
Here’s my pattern piece and my fashion fabric.
I grab some denim for my flat lining material and use my pattern to cut out the pieces I need. (Miscellaenous fact: most historical costumers will use coutil for their corset flat lining material but coutil is expensive, so I just use denim.)
Pin your fashion fabric to the flat lining pieces you just cut…
And stitch the 2 pieces together.
Now you just treat the entire thing as one piece when you assemble your garment.
And that was flat lining!