The side bars of a drawer are technically called "frames", their function is to have the drawer sliding on their rails. Watch the inside of your drawer: do the side bars (i.e. the frame) run vertically straight down to the bottom forming a 90° angle with the bottom plate? Or do the frames appear rather slanted and conical?
Wooden drawers and modern pull-outs made of steel normally have straight inner sides to the left and right and are straight on the front and back, too. We call these drawers "drawer frame type #1 (straight)".
If you notice that your drawer's frames are conically shaped, you will have to take into consideration that the measurable width at the bottom line is smaller than the width at the top. We call these drawers "frame type #2 (slanted)".
Our products compensate for a slight transition of slanted frame sides. However, if the difference between bottom and top width is more than 8mm, order "frame type #2 (slanted)".
Get more information about shapes of drawer frames and the manufacturers Blum, Grass, and Hettich etc. in our lexicon of drawers.
Drawer frame type #1 / straight (c.f. Blum Legrabox)
Drawer frame type #2 / slanted (c.f. Blum Tandembox)