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Adjustable tension screw: This type of tension system uses a screw with a wide slot that is easily adjusted by a coin. It does not require the need of a special tool or screwdriver.
Adjustable Tension Knob: This allows allows the stylist to simply turn the knob adjuster with their fingers clockwise to increase or counter clockwise to decrease tension. No tools are required
These are the two most widely found on scissors. That's not to say there aren't other variations.