Rashi is the Sanskrit word for a zodiac sign, and the rashi chart, the D1, is the chart everything else in Jyotish is built on. Astrolium draws it in whichever regional style you read in.
A rashi is one of the 12 signs of the sidereal zodiac, each exactly 30 degrees wide, running Mesha, Vrishabha, Mithuna, Karka, Simha, Kanya, Tula, Vrischika, Dhanu, Makara, Kumbha and Meena. They share names and rulers with the Western signs but sit roughly 24 degrees earlier against the stars, the offset set by the ayanamsa. The sign holding the Moon at birth, the janma rashi, is treated as the primary sign in a Vedic reading, where Western practice would reach for the Sun. Because Jyotish uses whole-sign houses, the rashi containing the Ascendant becomes the 1st house entire, so signs and houses are the same 12 divisions counted from different starting points.
The 12 signs and their lords
Each rashi is 30 degrees of the sidereal circle and carries a traditional lord: Mars rules Mesha and Vrischika, Venus rules Vrishabha and Tula, Mercury rules Mithuna and Kanya, the Moon rules Karka, the Sun rules Simha, Jupiter rules Dhanu and Meena, Saturn rules Makara and Kumbha. Classical Jyotish stays with the 7 visible planets plus Rahu and Ketu for rulership; Uranus, Neptune and Pluto have no place in the scheme.
Signs are also grouped three ways that practitioners use constantly: by quality into chara (movable), sthira (fixed) and dvisvabhava (dual); by element into fire, earth, air and water; and by the direction they rise, which is what decides whether a sign is read as strong on the Ascendant.
The exaltation and debilitation points sit at specific degrees rather than anywhere in a sign. The Sun exalts at 10 degrees of Mesha and falls at the same degree of Tula; Jupiter exalts at 5 degrees of Karka. A planet 1 degree away from its exaltation degree is not the same as a planet at the far end of the same sign, which is a distinction Western sign-level reading often flattens.
Janma rashi
The janma rashi, the sign holding the Moon at birth, carries more weight in Jyotish than the Sun sign does. It anchors personality description, it is the frame Gochara transits are counted from, and it decides when Sade Sati arrives, since that cycle is Saturn crossing the 12th, 1st and 2nd signs from the Moon.
Indian convention names a child from the syllables of the birth nakshatra pada, so the mansion and the rashi together do work in daily life that goes well beyond chart reading. If you only know your Western Moon sign, the sidereal one is usually the sign before it; the Moon sign calculator and the Vedic birth chart calculator will settle it.
Rashi and bhava
Because the standard Jyotish house system is whole sign, the rashi on the Ascendant becomes the whole of the 1st house, the next sign the 2nd, and so on around the 12. Sign and house stay locked together, which is why lordship arguments carry so much weight: the lord of a sign is automatically the lord of a house, and relationships between house lords produce the named combinations scanned by the Vedic yoga calculator.
Bhava Chalit, an equal division measured from the exact Ascendant degree, is used alongside the rashi frame in some lineages. It can place a planet in a different house without changing its sign, and it is a separate frame rather than a replacement. For the Western comparison, see whole sign houses and the house systems guide.
In Astrolium
The Jyotish workspace renders the D1 rashi chart in North Indian or South Indian style and keeps the two house frames apart on screen: whole-sign rashi houses in one tab, the equal-from-Ascendant Bhava column clearly labelled as the alternate. Divisional charts derived from the rashi positions, D9, D10 and D12, sit behind a switcher that pins the D1 facts so a varga placement is never mistaken for a rashi one. Everything runs on Lahiri, the value explained in the ayanamsa entry.