A four-in-a-row drop game with the lights off, where you track column heights to infer your rival's hidden discs.
Take turns dropping a disc into a column, where it settles on top of the stack. Column heights are public, so you always know how full each one is, but your opponent's disc colours are fogged. You see that a cell is filled without knowing whose it is, so you must deduce their plays.
The column drop is Connect Four, created by Howard Wexler and first sold under that trademark by Milton Bradley in February 1974.
The original element is partial fog: column heights stay public, but disc colours are hidden, so you know a cell is filled without knowing whose it is.
That is a carefully chosen amount of concealment. Connect Four is a solved game with a first-player win, and hiding colours while revealing heights keeps the geometry legible while removing the certainty — you can still count the shape of the stack, just not what it means.
Dropline Fog is an original free browser take on the four-in-a-row drop game, adding fog-of-war deduction to the familiar gravity grid. Play instantly, no sign-up.