Orthodontic mechanics are often described in terms of moving teeth, but an equal part of the work is preventing movement. That second half is called anchorage.
Force always acts in two directions
Pulling a canine backward along a wire applies an equal forward pull to whatever the elastic or spring is attached to at the other end.
If that attachment is the molars, they will drift forward. Nothing is faulty about the appliance; it is simply doing what any pair of connected objects does under tension.
Planning therefore starts by deciding which teeth are intended to move and which must be held, because both outcomes are being produced simultaneously.
Anchorage comes from resistance to movement
A tooth resists movement in proportion to its root surface area in bone, so a molar with several broad roots is far harder to shift than a single-rooted incisor.
Grouping teeth together increases the resistance further, since force must overcome the combined anchorage of the whole block rather than any single member.
Bracing an arch against the opposing arch through elastics recruits still more resistance, at the cost of relying on the wearer to use them consistently.
Space loss is the usual consequence of failure
Where teeth have been removed to relieve crowding, the resulting space is a budget to be spent on specific movements.
If the anchor teeth creep forward, part of that space disappears into unplanned movement and is no longer available for aligning the front teeth.
The correction is more mechanics and more time, which is why anchorage planning is done before the first appliance is placed rather than adjusted later.
Temporary devices changed what is possible
Small screws placed in bone provide a fixed point that does not move at all, because bone does not remodel around them in response to orthodontic-level force.
Anchoring against such a point removes the reaction force from the teeth entirely, allowing movements that once needed headgear or extractions to achieve.
They are minor procedures with their own considerations, including site selection and hygiene around the device, and they are placed only where the plan requires them.
Why the plan is individual
The amount of anchorage needed depends on how much space exists, how far the front teeth must travel, and what the profile should look like at the end.
Two people with identical crowding may need opposite strategies, because one benefits from the back teeth coming forward and the other does not.
Those decisions come from records, measurements and an examination, which is why orthodontic plans are not transferable between cases that look alike.