Fidelity offers three commission levels depending on how many trades you do per year and/or the total across all your Fidelity accounts. The default rate that everyone qualifies for is "Bronze" which costs $19.95 per trade. Ouch.
Since my retirement funds are now spread roughly evenly between my Fidelity (a rollover IRA), my Calvert account (another rollover IRA) and my current 401(k) I do not qualify for the "Silver" level which would only cost me $10.95 per trade. I would need $50,000 in household assets for my purposes (since I don't do at least 36 trades a year) in order to qualify for the lower commission and I have roughly half of that at Fidelity. The key word is household.
Fidelity allows you to consolidate accounts that are held within a household (which seems to mean "single address" to them) regardless of relationship between people. I think this is their way of reducing mailing (and printing?) costs, but they offer a nice carrot. Control over the accounts is not affected in any way and you just fill out a simple form declaring the household relationship and mail it in.
I just finished filling this out for LaLa and I -- so even though we are not married, by combining our Fidelity accounts into a "household relationship" we will soon qualify for the silver commission level, hopefully just in time to start next year's project: Investing!