Tall, thin, heavy books tend to fall over easily. Once one starts leaning, the rest often follow like dominoes.

Traditional bookends try to resist the force at the bottom, where the leverage is highest. When several books start leaning, even heavy bookends can slide or tip.

To solve this problem, I designed an adjustable bookshelf divider system mounted to the underside of the upper shelf. Instead of stopping books at the bottom like bookends, this system stabilizes them near the top, where the leverage is much smaller.

A slim rail holds movable dividers that gently stabilize books near their top edge.

The result is a minimal, discreet organizer that keeps books upright without bulky bookends. The rail sits close to the back of the shelf so most of the mechanism is hidden behind the books. Divider positions can be easily adjusted at any time.

How the Locking Mechanism Works

Inside the rail, the bottom surface has a zig-zag V pattern.

The divider base has a matching pattern.

When the divider is pushed down, the teeth interlock and prevent sliding.

When lifted slightly, the divider disengages and can move freely along the rail.

A secondary stabilizing platform prevents the base from tilting inside the rail while still allowing the vertical movement needed for adjustment.

Features

• Overhead anti-domino design
  Stops books near the top where the force is much smaller.
• Adjustable dividers
  Dividers slide along the rail and lock securely in position.
• Mechanical locking system
  The zig-zag V-pattern prevents movement when pressed down but allows sliding when lifted.
• Slim and discreet
  Mounted close to the back of the shelf so it remains mostly hidden behind books. The rail occupies only 14–15 mm under the shelf and the dividers are just 2 mm thick.
• Modular rail system
  Rails are printed in 249 mm segments (additional options: 166 mm and 83 mm) and connected with pegs to fit any shelf length.
• Customizable dividers
  Several decorative styles are included:
  • Leaf 
  • Honeycomb 
  • Waves 
  • Thunderstorm 
• Single or multi-color printing
  Decorative patterns allow color changes if you want them to stand out.
• Left and right versions
  Dividers are asymmetric and designed to protrude toward the books while the rail stays close to the back. Left and right variants allow the patterned side to face outward if desired.

Assembly

1. Print the rails
   Rail segments are available in 249 mm, 166 mm, and 83 mm lengths. 
2. Connect rails
   Use the included peg connectors to join segments to the desired length.
   Multiple tolerance options are provided so you can choose the best fit for your filament and printer. 
3. Plan the rail position
   Check shelf depth and divider size before installation. 
4. Mount the rail
   Attach it near the back of the shelf to the underside of the shelf above using screws or adhesive. 
5. Print dividers and stabilizing platforms 
6. Insert the divider into the stabilizing platform
7. Slide the assembly into the rail 
8. Install end caps
   Snap the end caps into the rail to prevent parts from sliding out. 
9. Adjust divider positions 
10. Insert books and enjoy a domino-free bookshelf.