Vail implement nine (9) keyer modes! In each mode, there are up to two (2) keys or paddles.
Straight / Cootie
Both left and right paddles transmit for as long as you hold them down.
Bug
Left paddle sends "dot" repeatedly while held, right paddle sends for as long as you hold it down.
Electric Bug (ElBug)
Left paddle sends "dot", right paddle sends "dit". Whichever was pressed most recently repeats.
Single Dot
Same as ElBug. If you press the left paddle while something is being transmitted, it sends a dit next, even if you let go immediately.
Ultimatic
Same as Single Dot. If you press the right paddle while something is being transmitted, it sends a dah next, even if you let go immediately.
Plain Iambic
Same as ElBug. Holding both paddles alternates between dit and dah.
Iambic A
Same as Plain Iambic. If you press and release either paddle while something is being transmitted, it sends what you pressed next.
Iambic B
Same as ElBug. If any key is pressed at the moment a dit or dah begins transmitting, that key will transmit once. The key that gets to transmit alternates between dit and dah.
Keyahead
Same as Ultimatic, but "remembers" as many keys as you press, just like a computer keyboard. If you are transmitting slowly enough, you can enter in a Q before the first dah has finished.
Thank-You
Vail's keyers were implemented based on the excellent descriptions in All About Squeeze Keying by DJ5IL. I am indebted to the author for writing such a lucid and helpful document: I was able to "fix" my iambic modes with no additional input beyond this PDF file!