Kaal Sarp dosh Yog is formed when all seven planets Sun, Moon, Mars, Mercury, Jupiter, Venus and Saturn are hammed between Rahu and Ketu. As you already know that the position of Rahu & ketu in rashi chart is always in front of each other, mean if rahu is placed in 1st house , ketu will be 7th house. Thus there are the possibilities in any chart to place rest of the planets in between Rahu and ketu, and this position is called Kaal Sarp dosh Yog. Its very important to consider the digrees of the planets place with Rahu or ketu to be sure about the existence of Kaal Sarp dosh Yog in natal chart, means if mars for example placed with rahu with 10 degree and rahus degree is less then mars degree, the Kaal Sarp dosh Yog dose not exist in that chart. Rahu and ketu degree must be more then the planets placed with them.
Generally Kaal Sarp dosh Yog gives struggle to native in all matters of life, related to health, wealth, career, profession, love, marriage, children and many other things related to our life, but it is very important to find out the exact effect of a Kaal Sarp dosh Yoga in the native horoscope. Is it giving bad effects and struggle in health, wealth, career, profession, love, marriage, children or other things related to our life? How would you know ? This can be judged by position of the Rahu and ketu in different houses of natal chart, for example if Rahu is placed in 1st house and ketu is in 7th house the person will have the difficulties in health and wealth as the 1st house indicates the health and the struggle of life of the native, thus if rahu is in 2nd house the person may have the struggle and difficulties with family relations, wealth, and other related things to 2nd house. Thus we can judge the effects of a Kaal Sarp Dosh Yog in 12 house of the natal chart and it make 12 types of Kaal Sarp Dosh Yoga.