So, we have seen so far that each planet owns (rules over or ruler of) some signs and gets Exalted or Debilitated in some signs. Now, there is another category of signs, which are called Mool Trikona Sign. So, if we try to arrange the good results for a planet in various signs then it can be said that a planet gives best results in exalted signs, good results in own signs and somewhere in between it gives better results in its Mool Trikona Signs. Lets see by example - Jupiter is exalted in Cancer (best results). Jupiter owns two signs, i.e. Sagittarius and Pisces. Out of these two signs, Jupiter is more comfortable in Sagittarius as the nature of Sagittarius and Jupiter are somewhat similar in comparison with nature of Pisces and Jupiter. So, Sagittarius will become Jupiter's Own sign as well as Mool Trikona sign. Here, Jupiter will give better results than other own sign Pisces. 

So, What is Mool Trikona Sign of a planet? 

Out of the signs which a planet own, in whichever sign it is more comfortable or whichever sign is closer to the nature of planet, that sign becomes its Mool Trikona Sign. Lets see what are the Mool Trikona sign for each planet and the reasons - 

1. Sun - Sun owns only one sign, i.e. Leo. Sun is King among planets and Leo (Lion) is King of Jungle. So, it fits to the character of Sun, hence, Leo is its Mool Trikona sign too. 

2. Moon - Here is one case of differences of opinion. Moon owns Cancer sign but its Mool Trikona sign is Cancer as per some astrologers while Taurus as per others. Moon is mind and emotions and Cancer is certainly the same thing. So, in my considered opinion, Cancer is better choice.. 

3. Mars - Mars rules over two signs, i.e. Aries and Scorpio. Now, Mars represents our will power, courage and action taking ability. Aries represents Self and people who are very active and go-getter kind of, closer to Mars characteristics. Hence, Aries is Mool Trikona sign of Mars. Although, Scorpio is also a sign ruled by Mars, but Scorpio is about occult science, research and transformation. So, Scorpio will remain Mars' own sign, whereas Aries Mool Trikona sign. 

4. Mercury - Mercury rules over two signs, i.e. Gemini and Virgo. Mercury represents logical thinking, quick decision making and analysis ability besides communication and marketing. Virgo represents analytical approach and being critical about things. Hence, Virgo becomes Mool Trikona sign for Mercury whereas Gemini remains its own sign. 

5. Jupiter - Jupiter rules over Sagittarius and Pisces. Jupiter represents religion, philosophy and literature etc and Sagittarius represents the same things. Hence, Sagittarius is Jupiter's Mool Trikona sign and Pisces remains Jupiter's own sign. 

6. Venus - Venus rules over Taurus and Libra. Venus represents relations, it represents wife for a man and it is main significator of marriage. Libra is sign of relationship. So, Libra is Mool Trikona sign of Venus. Taurus would remain as Venus' own sign. 

7. Saturn - Saturn rules over Capricorn and Aquarius. Saturn feels more comfortable at Aquarius because of its portfolio of networking, large organisation and huge buildings, hence Aquarius is Mool Trikona sign of Saturn.

Again, Rahu & Ketu, two illusionary planets, don't have any sign as Mool Trikona sign.