Many people are not aware that there are actually major differences between “spinet” pianos and other upright pianos. Spinets are the shortest of upright pianos (generally less than 40″ tall), and the small size has other compromises built in, but by far the most significant is what’s known as the drop action.
In all other upright pianos, the back end of the key pushes upward against the action mechanism, propelling the hammer toward the strings. In a spinet, as you can see in this picture, the back ends of the keys are actually attached to lifter rods, which drop down below the keybed, connect to the elbow and a special lever which transfers the movement to the action and the hammer.
A drop action is a somewhat more complicated and less efficient way of transferring energy from the key to the hammer, but at one time was considered a worthwhile compromise to accomplish the shorter height of the piano. Drop actions are rarely built anymore, but hundreds of thousands of them are still around.
And it pays to know what you’re looking at, particularly if you own or are looking for a used upright piano!