Two kinds of cleaning
Most people come in for a regular cleaning, which deals with plaque, tartar and staining above the gum line. Some people need a deep cleaning instead, which goes below it.
Which one you need is not a guess. It comes out of the gum assessment at the start of the appointment.
The gum assessment
The hygienist measures the pockets between your gums and your teeth, checks whether the gums bleed, and looks at your X-rays for signs of bone loss. Shallow pockets and firm gums mean a regular cleaning. Deeper pockets, bleeding, or tartar sitting under the gum line point to a deep cleaning.
A regular cleaning
Plaque and tartar are scaled off, the teeth are polished, and the hygienist will show you anywhere you are consistently missing when you brush. For most people this is the twice-yearly visit.
A deep cleaning
This is scaling and root planing. Tartar is cleared from below the gum line and the root surfaces are smoothed so the gum can settle back against the tooth instead of holding a pocket open. The area is numbed first, and it is usually split across two visits, one side of the mouth at a time.
After a deep cleaning
Gums that have been treated once need watching. Cleanings move to a shorter interval, often every three or four months, so the pockets do not deepen again. That is periodontal maintenance rather than a return to the six-month clean.