First MortgageFinancial Dictionary -> Mortgages -> First Mortgage
This is often referred to as "Primary Lien," which means the lender has precedence over all other mortgages in case of a default because it was the first mortgage.
So if at any time the borrower breaks the contract by not making payments, breaks any of the other agreed terms and is forced to the sell the house as collateral, the first mortgage lender will get what's owed to them before any secondary mortgages or loans.
For example Richard's house is worth $100,000 and he finances it with two mortgages, the first mortgage is $75,000 and the second mortgage is $15,000. If Richard defaults due to nonpayment and is forced through the foreclosure process, the lender of the first mortgage will get their full due amount from the sale and the second lender will get any excess. It might not necessarily cover it, but that's the risk they take for being a second lender.
Somebody will usually have a second mortgage to release equity in the home built up through the payment of the first mortgage, rather than having two separate mortgages to finance the original purchase. This allows for things like expansion and home repairs, but if it all goes horribly wrong it's the original loan that is repaid first.
First mortgage amounts vary a great deal from case to case and property to property and a person's credit history and income are taken in to account before lending them a mortgage. Some people are rejected mortgages and others are given a 100% mortgage. It just all depends on the borrower and the terms of the lender.