The number that converts one zodiac to the other
Ayanamsa is the name for the gap between where the tropical zodiac starts and where the sidereal zodiac starts. It is a single angle, and it does one job: subtract it from a tropical longitude and you have the sidereal longitude of the same point. Right now it stands at roughly 24.2° and it grows by about a degree every seventy-two years, since the equinox point it is measured from keeps precessing. Every Vedic chart ever cast has this subtraction inside it, usually invisibly. If you have wondered why software asks you to choose an ayanamsa before drawing a chart, this is what it is asking: which value of that offset should the conversion use.
Why there is more than one value
The offset is measured from the equinox, which is well defined, to the start of the sidereal zodiac, which is not quite. The sidereal zero point has to be tied to something — a specific star, a computed position, a historical date on which the two zodiacs are asserted to have coincided — and reasonable people have chosen differently. That is the entire source of the disagreement. It is not a dispute about precession, which nobody contests and everybody measures the same way; it is a dispute about where to put the origin. Once a school fixes its origin, its ayanamsa for any date follows by straightforward calculation, which is why the competing values track each other exactly and never diverge further over time.
Lahiri, Raman and KP
Lahiri, sometimes called Chitrapaksha, is the value adopted by the Indian government’s Calendar Reform Committee and is the standard used by the national ephemeris, most published panchangs and most software. It is what GrahaVaani uses, which means every chart and reading on this site is calculated against it. Raman, associated with the twentieth-century astrologer B. V. Raman, sets a slightly different origin and produces a value a fraction of a degree from Lahiri. KP, from the Krishnamurti Paddhati school, likewise sits a fraction of a degree away and is used almost exclusively by practitioners of that method. All three are small variations on the same roughly 24° offset — none of them is a different kind of measurement.
When the choice actually matters
Almost never for a sign. A fraction of a degree cannot move a planet across a 30° boundary unless it was already sitting within that fraction of the edge, so for the great majority of charts every ayanamsa in common use returns the same signs. It starts to matter as the divisions get finer. Nakshatras are 13°20′ wide and their padas 3°20′, so a position near a pada boundary can genuinely land differently depending on the value chosen — and since the birth nakshatra sets the dasha timeline, that can shift period dates. The same applies to the finer divisional charts and to any technique that reads a planet to the degree. The practical rule is to pick one and stay with it rather than comparing outputs across systems.
What this does not mean
A disagreement of a fraction of a degree among ayanamsas is sometimes presented as evidence that the whole sidereal framework is arbitrary. That reads more into it than it holds. Precession itself is not in question, the mathematics of the conversion is not in question, and the three values in common use agree closely enough that most charts come out identical. What is genuinely unsettled is a definitional choice about the origin, made once by each school and applied consistently ever since. If you are comparing a chart from one site with a chart from another and the positions differ slightly, the ayanamsa setting is the first thing to check — and usually the whole explanation.