The part that joins the upper of a shoe to the sole is called the midsole.

Here's a breakdown of the different parts of a shoe:

* Upper: This is the part of the shoe that covers your foot, made from materials like leather, fabric, or synthetic materials.

* Midsole: This is the layer between the upper and outsole, often made of foam, rubber, or other cushioning materials. It provides support, cushioning, and flexibility.

* Outsole: This is the bottom part of the shoe that comes in contact with the ground. It provides traction and durability.

So, the midsole is the key component that connects the upper to the sole, creating the structure and functionality of the shoe.