No. of Recommendations: 2
Here are the respective books at E*Trade and IB for a selected issue, first sell-side and then buy-side, as they appear in drop-down menus.

Bids at ET Bids at IB
110.175x250 110.600x84
110.000x50 110.000x675
110.000x225 109.875x200
111.000x475 109.800x100
109.875x50 109.491x100
109.490x100 109.490x100

Asks at ET Asks at IB
110.750x40 110.750x80
110.995x44 110.995x176
111.000x30 110.000x1040
111.173x4 111.075x100
111.475x100 111.162x4
111.500x93 111.250x93

Interpreting those numbers.
IB lumps the lots by price. E*Trade shows a more granular book and partitions the lots by desk. ET shows no more than six deep. IB reports whatever depth it needs to. You can assume that the rounds lots (bids or asks of 100) were made by market-makers doing their pledged duty “to maintain a fair market.” But they will back away from those bids and asks anytime they choose and then deny it by saying, “The market changed. That’s why you didn’t get a fill on your order.” You can file a protest. But you’ll likely lose the case. The book at either broker is constantly changing, especially if the issue is liquid. The data I report were copied from screen shots. By the time I typed this, the market had moved. Lastly, as you can see, they are not quoting from the same network of dealers. There are price overlaps, but there are differences that go beyond in-house inventory and cannot be explained by time-stamping.

Why might this info matter?
For most part, it really doesn't. It's just geeky details that can be ignored *unless* you're trying to get into a position at a smaller size than the minimum-purchase size for the inside market (aka, the national best bid and ask). In that case, by going into the book, you can generally find a desk willing to sell in a smaller size, albeit at a higher price. Depending on the issue you're trying to buy and/or how you intend to manage your risks, it can be a smart move to "pay up", so that you can reduce your exposure.

Standard Disclaimers: The preceding info is believed to be correct but must be checked before being used to make financial decisions.
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