If I'm not mistaken trading (commissions) is not where they make their money, which is why most of them offer free trades now in some capacity to compete. If th

If I'm not mistaken trading (commissions) is not where they make their money, which is why most of them offer free trades now in some capacity to compete. If they don't you call them up and tell you are going to someone else and they will give you free trades. They make their money by having your money in their account. This gives them credit with other banks and allows them to trade on margin or borrow or lend and charge interest.