These are sets which reproduce the method of erecting buildings by first constructing a girder framework and then attaching wall and roof panels to the frame. They generally produce flat roofed stuctures whose shapes are effectively collections of cubes. However, with a variety of panel types and enough parts, quite pleasingly complex buildings can be produced.
I should say that I am using the term Girder and Panel sets here to refer to a generic type, one example of which is the range of sets of that name produced in America by Kenner, and later owners of the trade name. Kenner sets also appear to have been sold under the Chad Valley name in Europe.
These sets contain upright and cross girders to form the frame; the former have lugs at the bottom which fit into either holes in masonite base boards (for the ground floor) or sockets at the top of other uprights for subsequent floors. Cross girders have tiny V-shaped extensions at their ends which drop into cut-outs at the top of the uprights. The walls are formed by thin flexible panels of various designs with three holes which 'snap-on' to lugs on the girders. In earlier sets the panels are lightly moulded with walling rendered in a sort of pyramidal tiling. Panels have various combinations of wall and window elements and walling comes in a number of colours. Later sets had flat panels with the details printed on. Roofs are drop in panels, either plain or with hemispehrical dome skylights. Later still, the sets produced under this name by Irwin include diagonal girders which create some rather interesting buildings.
Kenner also produced a range of sets called Build-A-Home which used the same construction technique to build american domestic and suburban architecture. In this case the wall panels are brick and clapboard effect and pitched roofs are produced with angled girders. These sets of course violate my categorisation because they do not imitate the actual construction method of the buildings represented. Oh well, no system is perfect.
There are, as far as I know, few sets which fit into this category and the other main one is Arkitex. This is a british product, produced by Triang in two scales, 00 and 1/42. Here the girders are interchangeable and are connected to each other and the plastic bases by means of six-way connectors of this shape:
whose arms fit into sockets at the ends of the girders.
Arkitex panels are rigid and far more robust than the Kenner ones. They attach to the frame by hanging off the upper girder and came in three basic designs: plain wall, windows with coloured panels beneath and glazed wall with double doors. Accessory packs could be bought with wall panels of different textures, although all wall parts are white. These sets also produce more finished looking buildings than Kenner as they fully clad the framework by having parts to cover the corners and roof edges. Drop in card panels form the roof. Complex shapes often require special parts which where not usually contained in the standard sets, making Arkitex less flexible than Kenner.