Minecraft wiki entry for Scaffolding, associated text: The placement of scaffolding is different from any other blocks in the game. When pressing use on the side of a scaffolding, the scaffolding is added on top of the highest scaffolding in that location, allowing the player to build the scaffolding structure higher on the ground.‌[Java Edition only] Pressing use on the top face of the scaffolding adds the scaffolding to the side, depending on the direction the player is facing.‌[Java Edition only] When trying to place a scaffolding inside of another scaffolding, the scaffolding is added on top of the highest scaffolding in that location, allowing the player to build the scaffolding structure higher on the ground. While sneaking, pressing use on the top face of the scaffolding places the new scaffolding on top of the targeted one if the top is available, otherwise it is placed under the targeted scaffolding if possible. Pressing use on the side while sneaking places it on the side like placing other blocks.