This is a basic and very important of Astrology. It is more utilized in Western Astrology but I see no reason why it should not be used in Vedic Astrology more often. Actually, there are many Vedic Astrologers who use it but it is not as frequently used by others as it can be used. So, let’s cover this concept on following points – 

Concept of House Cusps. 

Popular House Cusps. 

Planets on House Cusp Degrees. 

Results of Transits. 

Conclusion. 

Let’s cover all the points one after the other – 

Concept of House Cusps – As we know that a horoscope is divided into 12 houses. As per the rising sign at the time of birth, signs get moulded in the houses. So, if someone is born with Cancer Ascendant at the time of birth, his 1st house will have Cancer sign, 2nd house will have Leo sign and onward. A Zodiac Sign is 30 degrees long, so does it mean that 1st house of this person’s chart will begin at 0 degree Cancer and end at 30 degrees Cancer. That’s where the concept of cusps comes into consideration. As we know that Ascendant changes within 2 hours. So, if Cancer has just started rising over India right now, a Child taking birth at the moment will have Ascendant/1st house as Cancer and Ascendant Degree as 0 degree as Cancer has just risen. So, for this child, 1st house will be actually from 0 degree Cancer to 30 degree Cancer. But then a child takes birth after 10 mins. Now, if Cancer has covered 5 degrees at Ascendant, then child will still be a Cancer Ascendant/1st house but Ascendant Degree will be 5 degrees. Now, the 1st house cusp for this 2nd child will begin with 5 degrees Cancer to 5 degrees Leo, 2nd house cusp will be from 5 degrees Leo to 5 degrees Virgo and onwards. So, House Cusps are imaginary "boundary lines" for the Houses. Since positions in the horoscope chart are measured by degrees within the Signs, the House cusps do not always start at zero degrees of a Sign, but may start at any degree within a Sign. Similarly, if someone is born at Cancer 10 degree or 15 degree, he will remain Cancer Ascendant/1st house but 1st house cusp will be at 10 degree or 15 degree, as the case may be.

Popular House Cusps – Western Astrologers normally pay much emphasis on House Cusps for reading of charts and predictions, hence they have given importance to some specific House Cusps like Ascendant Cusp/1st house Cusp, Descendant Cusp/7th house Cusp, Imum Coeli Cusp/4th House Cusp and Mid-heaven Cusp/10th house Cusp. We can relate it to Kendra Houses (Houses # 1, 4, 7 & 10) but all that matters here is Cusp Degree. If someone is born with 25 degree at Cancer Ascendant, then his Mid-heaven Cusp will actually begin at 25 degrees of 10th house and will end at 25 degrees of 11th house. So, it will include more area of 11th house than 10th house. That’s where, Ascendant Degree plays an important role in deciding the Cusps. 

Planets on House Cusp Degrees – Now, the main effect of House Cusp is at the dignity of planet and its results in MD. Suppose someone is born with 10 degree Aries Ascendant. Now, any planet in his chart, placed in any house or any sign, between 7 degree to 13 degree, i.e. plus/minus 3 degrees from the Ascendant Degree of 10 degrees, will be very potent and its MD can bring great results. Closer the planet is to ascendant degree, more powerful it is. Few exceptions to this rule are if planet is debilitated at exact degree, like Sun is at 10 degrees in Libra, then it will have its debilitation effect too, or a malefic like Rahu or Saturn will give its malefic effect too besides getting potent as close to the Ascendant Degree. So as always, everything is in addition in Astrology, nothing cancels anything. Another example is if someone is born with Gemini Ascendant at 2 degrees, then any planet in any house/sign (keeping exceptions in mind) which is around 29 degrees to 5 degrees will become extremely powerful and its MD can bring great results. 

Results of Transits – And this is also the reason why annual transits of planets give their results to different people at different times. As everyone will have house cusps beginning at different degrees, transiting planets will hit the cusp at different times and that’s why people experience results of same planet’s transiting at different times. Like, Saturn is about to enter Sagittarius. Now, someone born with Sagittarius Ascendant or any Ascendant in early degrees like 0-5 degree, will see the impacts of Saturn’s transit into Sagittarius, in the house where Sagittarius falls as per ascendant, as soon as it begins. But if someone is born with ascendant in later degrees, he will get results after a considerable delay or may be even after Saturn crosses the half-way into Sagittarius. 

Conclusion – This is again the basic info on house cusp and I am sure that much more info is available online or otherwise. I nowhere claim to be master in this concept. Articles of Western Astrologers are going to help in this great deal. So, please explore and if you find any further useful info then please feel free to share with me.