While considering a marriage alliance, the Hindu custom does not allow the tying of the knot until the ten poruthams are studied and matched for the couple who desires to be joined in matrimony. Only in cases of a love affair or marriage contemplation between a boy and a girl of an older age group, the application of the poruthams can be omitted.
Poruthams or kootas are based on the birth stars and the janam rashi of the boy and the girl in question. There are ten in number and each is considered for the marriage alliance. Each porutham is significant according to different aspects of a person's mind, body and personality, and the effects of each are considered one by one.
The Ganam porutham is the second amidst the ten poruthams (the Dinam being the first porutham) and takes into account the matching of the temperaments of both the boy and the girl to assure compatibility in their sex life after marriage.
Ganam Porutham is differentiated into three types. These are the Deva Ganam, Manushya Ganam and Rakshasa Ganam, each of them represented by nine stars. People under the Deva Ganam are good natured, soft spoken, generous, well behaved and affectionate. They consider a good character and a life of high values of great importance in their life. Those under the influence of Manushya Ganam possess a character with mixed qualities. In other words they have both good as well as bad qualities in them; they have good behavior but can also display anger at times and even enmity. Under the category of Rakshasa Ganam come those people who possess an outright bad behaviour; both in their speech and their actions. They are rude, aggressive and rough to anyone and everyone; they are unpleasant with others and find it difficult to cooperate with them and think they are always in the right. Only rarely are they found to behave well with others.
If both the boy and the girl are of the deva ganam porutham, they are said 'to agree' and this match is considered as 'every good' (ati uttamam); if both are of the manushya ganam, they are also said 'to agree' and it is also considered as 'good' (uttamam) but if both the girl and the boy are of the rakshasa ganam, the match is not recommended because the marriage will not be a happy one and will not last long thus destroying the very reason why the poruthams are made even though the match is said to be 'uthamam'.
If the girl comes under deva ganam and the boy under manushya ganam or vice versa, their match is considered as a secondary match or 'not bad' (madhyam). If the boy is of the rakshasa ganam and the girl is either a deva ganam or a manushya ganam, the match is considered to be mediocre or 'madhyam' but if the girl is of the rakshasa ganam and the boy is of any other gana, the match is considered 'athamam'.