Chart ruler
The chart ruler is the planet that rules the sign on the Ascendant, and classical astrology treats it as the significator of the person themselves.
Because it stands for the native, its condition matters more than most: the sign and house it occupies, its dignity, whether it is in sect, and whether it is free of the Sun's beams. A well-placed chart ruler is read as a life whose own affairs run under their own steam.
It is not the same as the strongest planet in the chart. The almuten is calculated from weighted dignities; the chart ruler is decided by one thing only, which sign rises.
Frequently asked
How do I find my chart ruler?
Look at the sign on your Ascendant and take its traditional ruler. A Leo Ascendant gives the Sun; a Scorpio Ascendant gives Mars.
Is the chart ruler the same as the dominant planet?
No. The chart ruler follows from the rising sign alone, while a dominant planet is worked out from weighted dignity and placement.