It is as it were the 'flesh' behind everything that really needs to be stitched firmly on canvas. Moreover, this reinforcement triangle can prevent sharp objects from sticking through the fabric.
The material is the layered and coated PVC (two layers of heavy PVC and a polyester fabric in between) of no less than 630 gr/m². This material is also known as Bisonyl. Also known as heavy truck tarpaulin.
11 x 11 x 16 cm