Since others have asked me to comment on this, I will be glad to offer the following. You'll be able to tell exactly when a Specialist is participating in certain "prints" and when he is "likely" to have participated in others. First example. When you see the bid or offer change significantly, without the other side changing at all...and then a block trade ("print") go up at or near the recently changed quote price, you can pretty much be assured that the Specialist took part in a negotiated trade. This activity indicates that the quote price must be changed so that the trade price falls within the (new) NBBO as required by regulations. These block trades are always 10,000 shares or more in the stocks I trade. Second example. When you see the quotation size go to 1 x 1 (indicating a closing of the book, allowing the Specialist to sort out the orders in hand without concern for new orders (at least for a few seconds). This will likely indicate that the Specialist is about to "print" a block (or a few blocks) around the last sale. Either he is "cleaning up orders" with his participation, or he is accomdating orders, with limited participation. I teach my people how to read the running tape, live and real time whenever possible so that we can respond to the activity outlined above, and possibly participate. After all is said and done, a review of T&S will show, if nothing else, what we missed out on. I also teach how to prepare for these anomalies before they happen (at least in our core stocks). Double prints are simply the "matching" of trades between a broker and the Specialist or book. Go to the NYSE and get a copy of their rule book if you like, but the basics are pretty simple. The Specialist can only "participate" not "initiate" upticks and downticks (thus the appearance of double prints, etc. at times). The Specialist is generally "accomodating" orders rather than speculating on price movement. I hope this helps... Don