Am I correct in thinking that when a trade occurs, the Ask and Bid price are the same, the Mark, being between the Ask and Bid would therefore also be the same as the Ask and the Bid, and with the trade being completed, the Last Trade price would also be the same?
Hi @MoreIsNotEnough,
When a match is made, the next Bid and Ask prices in the order stream are what you will see in quotes while Current is that last match made.
You would never see Bid and Ask equal since those orders would be filled. If that were to happen, there might be one seller or buyer with an order posted. The opposing side would be zero.
Does that help you?
Gene
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I think that tells me that they are close enough for my purpose.
To go into my concern, for years I’ve been trading ETFs, by placing conditional trailing stop limit orders with Schwab using Street Smart Exchange. Their order screen is an easy to follow dialogue. SSE is going to be replaced with Thinkorswim which is not so intuitive. In TOS to place an order similar to the ones I place in SSE, I need to specify a limit offset, linked to a choice of prices. I think linking the offset to the Bid price (for a buy and Ask for a sell) is my best choice, but after thinking about it for a couple of days, I think it doesn’t matter because Ask, Bid, Mark and Last would all be the same when the trade that is triggered by the specified condition takes place. I’ve been experimenting on TOS with paper money, and so far it appears to bear this out.
Sorry you asked? 
The trading situation isn’t as simple as some have suggested. Just because the inside bid or ask is or isn’t the same doesn’t mean that trades are or aren’t occurring, nor that such trades as might have happened are being posted. E.g., odd lots aren’t posted, nor can most retail investors/traders see what’s happening in the dark pools.
Also, no matter the tradable, the spread is constantly varying, as is the depth of the order queue. Also, also, there’s the matter of trades that happen in extended hours.
So, what’s a workable solution for the typical retail investor? Fugure out at what price you’d be willing to buy or sell and write your orders accordingly.