The Lagna chart and the first house
The lagna chart is the chart drawn with the Lagna as house 1. It is the main chart in Jyotish and the one most readings start from.
What it is
In the lagna chart, the sign of the Lagna is house 1, and the other houses follow the signs in order. The chart is the basis of the house, lord and graha placement reading.
How it is worked out or read
This site's birth chart is the lagna chart in whole-sign houses. The houses are sign-based, so a planet's house is the sign it occupies, counted from the Lagna sign.
Where it appears on this site
The lagna chart is what the birth chart tool shows. The Moon chart article describes the other main chart.
The chart built on the Lagna
The lagna chart is the chart drawn with the Lagna as house one. It is the main chart of Jyotish and the one from which most readings begin. Every house, every lord and every graha's house in the lagna chart is counted from the Lagna sign, so the chart is the base on which the rest of the interpretation stands.
Because the lagna chart is the default, a reader who says simply that a planet is in a house is normally speaking of the lagna chart. The other charts, such as the Moon chart, must be named when they are used.
What the Lagna chart shows
The lagna chart shows the Lagna sign, the sign and house of each graha and the houses as sign-based divisions of the zodiac. It is the view from which the classical rules for the body, the career, the marriage and the fortune are usually stated. Reading the lagna chart means reading all twelve houses in order, each with its sign and its lord.
The chart also shows the relation between the houses and the grahas. A planet in the fifth house from the Lagna has a different relation to the rest of the chart from a planet in the first, and the lagna chart makes that relation explicit.
Using the lagna chart with other charts
A careful reader uses the lagna chart as the main reference and the Moon chart as a second view. Where the two agree on a theme, the reading is more consistent. Where they differ, the reader should state both and say which rule uses which reference, since the classical texts assign different subjects to each.
The lagna chart is also the chart used by the birth chart tool on this site, and the checks use it for the Manglik and Kaal Sarpa counts, while the Sade Sati rule uses the Moon sign.
The limits of one chart
A single chart is a snapshot of the sky at one moment in one place. It does not show the chart at another time, nor the way the sky changes during a life. The dasha and the transits add that dimension, and the reader should describe the lagna chart as a fixed reference that is read together with the timing tools rather than on its own.
Reading the lagna chart in order
A useful habit is to read the lagna chart in strict order, from the first house to the twelfth, and to note the sign, the lord and the grahas of each house before moving on. This prevents the reader from jumping to dramatic placements and keeps the whole chart in view.