Final answer:
The thumb is placed in the thumb grip of the shears, permitting the opposition movement that is vital for control and precision in haircutting.
Step-by-step explanation:
When holding haircutting shears properly, the barber places the thumb in the thumb grip of the moving blade. Haircutting shears typically consist of two blades, one will be moving and one will be stationary. The thumb grip is located on the moving blade, allowing the barber to control the opening and closing of the shears while cutting hair. Proper hand placement and grip are crucial for precise and comfortable cutting techniques in hairstyling.