There are 27 nakshatra counted in the Jyothisha.

There are 27 nakshatra counted in the Jyothisha. 

It starts from Kritika and ends with Bharani,  in the Vedas.

In Jyothisha, it starts from Aswini and ends with Revathi. 

In marital compatibility, the following pairs must be avoided. They are,

Aswini----Jyestta.

Bharani----Anuradha

Kritika---- Visaakha.

Rohini----Swathi.

Mrigaseersham---Chithira----Dhanishtta.

Ardra-----Sravana

Punarvasu----Uthra Ashada

Pushya----Poorva Ashada

Aslesha-----Moola

Makha-----Revathi

Poorva Phalguni-----Uthara Bhadrapada

Uthara Phalguni----Poorva  Bhadrapada

Hastha----- Shatabishak.

To elaborate, if the bride's nakshatra is, say, Pushya, the groom's nakshatra should not be Poora Ashaada and vice versa too.

This is discernible from the above set. 

This is called " Vedha". It harms marital harmony, health and financial status. The prosperity of the progeny may also be affected in some cases. 

However, there are several exception to this Vedha rule. 

If the Vedha is found before matching checked, it is often advised to reconsider the proposal. 

If unconditional love exists between the bride and groom, then, if is advised to offer prayers at the Lord Mahavishnu Temple where the manifestation of the deity is either "Koorma" or " Varaaha". 

This will cancel the Vedha demerit.