Ladder Jacks are an economical type of equipment (special brackets) used to convert a couple of ladders and one or more planks into a staging.
Over the rail and rung (as in the next picture) This older style jack takes advantage of the rail strength of the ladder, thus providing a stronger overall staging than "over the rung only" jacks. Because of the strength advantage here, this may be the preferred method when only "light duty" ladders are available, or when the staging will be required to carry a heavier load.
These "over the rail and rung" jacks are more of a hassle to use than the "over rung only" type, they can sometimes be difficult to get into position and then the planks must be slid into position from their ends.
When jacks are used on the outside of ladders (above right) a third ladder is usually required to access the staging
Over the rung only are often made of aluminum and are of a simpler design than the "over the rail and rung" type, thus they are lighter weight and quite easy to use. With the modern designed jacks, the plank can be set directly onto the brackets without sliding it in from its end. The "over the rung only" will have more limitations than the over the rail and rung. "over the rung only" they may require ladders of a certain minimal strength, and will usually have lower load limits than the "over the rail and rung"
Most ladder jacks are quite versatile, and can be mounted so that the staging planks are either on the upper or the under side of the ladders.