Ounce and Troy Ounce?

In researching gold prices some websites post current prices in just 'ounce' and some in 'troy ounce'. That is what brought me to this website. As far as gold is concerned the 3 ounce difference means alot. I am trying to find out what the difference in the two meanings is and if possible, is all gold weighed in troy ounces and the troy is just left off. I have quite a bit of scrap gold I am going to sell and I want to at least sound like I know what I'm doing so I don't get cheated too bad. Know what I mean?
Thanks for any help anyone can give me.

[Robert Fogt]

The troy ounce is traditionally the unit used in pharmacies and in precious metals such as gold.

Currently all stock markets quote prices for precious metals in the troy weight system, so it is still in very common use.

I would venture that all gold labeled in ounces is troy ounces. But since the troy ounce is slightly larger, I dont doubt there are some people out there who will scam people by selling in the avoirdupois ounce. But I doubt that is legal, I will take an educated guess and say you have to weigh it using the troy system or specify otherwise.

The Troy weight system, introduced somewhere around the year 900, is as follows:
1 troy pound = 5760 grains
1 troy ounce = 1/12th troy pound (480 grains)
1 troy pennyweight = 1/20th troy ounce (24 grains)
There is also the apothocarie system where:
1 dram = 1/8th troy ounce (60 grains)
1 scruple = 1/3 drams (20 grains)

The avoirdupois weight system, the more common system used in the US and introduced in England around the year 1300 is as follows:
1 pound = 7000 grains
1 ounce = 1/16th pound
1 dram = 1/16th ounce