I asked something similar yesterday. I used to think it was a shuffle between mms, but last week I placed and order for 23000 @ .0435. It was immediatly followed by an order of 23000 @ .043.
The second order came within a second so it had to be software driven.
Do short orders display on the T&S also? Would it make sense for MMs to short a stock in blocks equal to the orders of other traders? Long term it doesn't matter but I am curious.
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