It could be a few different things:

1. electronic volume vs pit volume vs combined volume. Depending on your data source, these all might be different

2. The migration of trading moving from pit to electronic

3. Change in what constitutes a "contract." Back in the pit days, when you had to phone in orders, you'd say "Buy 5 Christmas Corn at the market" even though this was 1 contract (5,000 bushels). Maybe - I don't know - they changed from recording volume from 5 for each contract to 1.

4. Maybe the rule for counting volume with spread trades (like calendar spreads) changed.


My guess would be on pit/electronic recording. Note you also see weird volume things with Beans and Wheat, too.

I'd ask the CME, they should be able to explain.

muttoez 

The below is the comment attached to corn from my data provider CSI. Prior to 19980101, Volume and Open Interest are given in 1000's of bushels rather than contracts

Ah Ha........I knew it would be something like that. The numbers even fit together. Thank you for that!