There is a very good article about these in a back issue of IOMNHA Soc. Magazine Vol X No. 2 pp3-16 by J. R. Roscow, called Manx Marriage Contracts: 1600-1736.

It starts "A Manx marriage contract was an agreement between the families of the groom and the bride, setting out in detail how much of the land, animals and other property would belong to whom, and when the transfer would take place".

What is particularly useful is that both parents of both bride and groom were often named at a time when it was quite difficult to find the wife, unless she was named in the husband's will. When she died before him pre-1700, and if there was a decree (often underage children), or if she left a will, this was usually under her maiden name. It is only in the body of the will or decree that you can see that her children and husband had a different name - so no way of finding her will in an index if her name wasn't known.

You could buy a photocopy of the article from the Manx Museum library, if staff or someone else will copy it for you. I bought my copy in 2003 - you need to sign a copyright form.
http://www.isle-of-man.com/cgi-bin/interests/genealogy/fhs/bulletin/index.pl?read=5773

Sue