Mark, mid, last
Last is a historical print. Mid is a cartoon between bid and ask. Mark is the venue’s fair for margin — sometimes a model. Your fill will be on the bid or ask unless you get lucky inside.
Terminal chains show venue marks for research. They are not your fill.
Width in vol points
A $20-wide ATM BTC option might be a tiny vol spread. A $2-wide far OTM might be several vol points. Convert mentally: if you cannot get a bid/ask IV, you cannot compare wings.
Skew and term desks need honest marks; garbage in the wings poisons the smile.
Size and flickering books
One-lot bids are not a market. Watch whether size refreshes or vanishes. Crypto options books can look fine until you lift 20 lots.
Tape on /t/{asset}/tape shows what actually traded. A strike with OI but no tape may be stuck inventory.
Limits versus marketable
Limits protect you from paying the panic ask. They also mean you may not get filled through the event you cared about. Marketable orders buy certainty at the spread’s expense.
See limit-vs-market.
Multi-venue marks
Two venues can show different mids because of index, fee, and who is allowed to trade. That is research (arb-guide), not an instruction to hit both bids.
Book desk /t/{asset}/book is built for BBO comparison.
Practice this on OptionsMatch
/t/btc/book and /t/btc/chain. If the banner says mock or partial, do not treat width as live.
Educational market-structure only.