A trammel hook is an adjustable hook used to suspend objects at variable heights.

Trammel hooks may be used to hold a pot or kettle

Kettle

A kettle, sometimes called a tea kettle or teakettle, is a small kitchen appliance used for boiling water. Kettles can be heated either by placing on a stove, or by their own electric heating element.- Stovetop kettles :...

 over a fire while cooking, allowing the height of the pot to be easily changed. Thus the rate of heating can be controlled.
They can also be used to hold candle

Candle

A candle is a solid block or cylinder of wax with an embedded wick, which is lit to provide light, and sometimes heat.Today, most candles are made from paraffin. Candles can also be made from beeswax, soy, other plant waxes, and tallow...

s, where changing the height makes the area lit wider or narrower.