A graha is not read in isolation but through the several tables that make up a Panchanga and a chart. It rules Wednesday in the seven-day week, which in the Indian tradition as in the European one is named for the seven visible grahas in the order of the planetary hours. It holds 17 years of the Vimshottari dasha cycle, and it is lord of three of the twenty-seven nakshatras, Ashlesha, Jyeshtha and Revati, spaced nine apart so that the nine grahas cover the twenty-seven exactly three times over.
The Vimshottari dasha system divides a notional lifetime of 120 years among the nine grahas in a fixed order, Ketu, Venus, Sun, Moon, Mars, Rahu, Jupiter, Saturn and Mercury, with fixed lengths that sum to exactly 120. Which period a person begins life in is determined by the nakshatra the Moon occupied at birth, and how far through that period, by how far through the nakshatra the Moon had travelled.
In the scheme of dignities it is exalted in Virgo, which is also one of its own signs, and in its fall in Pisces, and dignity is what the classical rules for strength are built on, since a graha in its own sign or its exaltation is read as able to act and one in its fall as constrained.
Mercury is the only graha whose sign of exaltation is also one of its own signs, which collapses two of the usual dignities into one and makes its treatment slightly irregular throughout the corpus. It is also never far from the Sun in longitude and is therefore frequently combust, a condition the tradition reads as weakening, so a rule requiring a strong Mercury is a rule that will often be qualified.