A completely invaluable and frankly amazing, sewing tool. In corset making, I use this sewing guage to easily measure the width of boning channels, spaces between channels, spaces between eyelet holes, and all sorts of other 'fiddly' measurements where a tape measure or ruler is no good.
In dressmaking and craft sewing it has many more uses - the button shank tool is genius! Take a look at this instructional video .. It's the sewing tool that keeps on giving!