I read somewhere that a 1/2 stopper for No Trumps is Qx or Jxx. So if I bid 4th suit forcing I need half a stop in the fourth suit and am asking partner if he/she has a half stop. If I have Qx or better and partner has Jxx or better (or vice versa) then our opponents can't run a long suit off the top.
In a book called "25 Acol Conventions You Should Know", Sandra Landy et al say that after a takeout double responder needs 1 1/2 stoppers to bid 2NT (10-13 HCP) or 3NT 14+ HCP. I know what a single stopper looks like and a double stopper, and now know what a 1/2 stopper looks like, but what does 1 1/2 stoppers look like? I presume it means any holding that, opposite Jxx or Qx, can provide a stop, then lose the lead, and then provide a second stop, but is it definable more specifically, as per the 1/2 stop?