Lugs

The majority of steel frames are brazed. There are two types of brazing. Fillet brazing welds tubes directly to each other by creating a buildup of the brazing metal at the joint between tubes. It is lighter than lugs, however it is not as strong. Tubes fitted to lugs must be mitered carefully to ensure a proper fit. Silver brazing allows a tighter fit and lower brazing temperature, which requires more control of the flame but lessens the risk of deforming the tubing from overheating. Bronze brazing is more common and less expensive. If a frame is well made, the metal used will not significantly affect the bike's strength or performance.
In the early days of lugs, they were sand cast. This method created bulkier lugs that required a lot of finishing work to get to the right size and to clean up. These early lugs were little more than pipe fittings, with square ends. Although they were adequate for early bikes, the evolution of steel tubing led to lighter, thinner tubes. Suddenly these new bikes built with old lugs would fail at the edges of the lugs. The area where the lug joined to the tube created a 'stress riser,' a place where force was concentrated.
Builders discovered that by creating curving lugs, they could distribute the stress along a larger area of the tube. New methods of creating lugs were in use.
Stamped lugs began as a single sheet of steel which was stamped and formed into a lug, and then welded. The downside to this was that the builder had to work to clean up the weld area to make it presentable. The angles of stamped lugs can be slightly altered by the builder.
Investment casting is a method of lug creation in which lugs are made by pouring metal into a ceramic mold. The resulting lugs are more precise and can be more detailed.
Most builders refine the lugs before they put them on a frame. They may alter the curves of the lug by filing, and generally thin the lug out. New lugs are thicker and heavier than they need to be. If you look on a low-end bike the lugs are typically thicker than on a high end bike. Another thing to examine on a bike's lugs is the quality of the brazing or workmanship around the edge of the lug. Occasionally, I find a bike which has a lug that is coming apart from the tube. I also see lugs were a buildup of bronze has occurred at the edge of the lug. A third indicator of poor quality or workmanship is the presence of bubbles at the edge of the lug. Frames with these problems may be perfectly good to ride for many years to come, despite their flaws, because of the strength of the lugged method as a whole. I have also seen a Trek where the headtube sheared off just above the lug, probably due to poor brazing (overheating?), but the workmanship looks deceptively high quality. Below is a picture of a headtube before paint is applied, with bronze visible.

There are a few styles of lugs which can be easily recognized. The first of these is nervex, which are intricate and easily recognized. They are in the picture to the left.
Also in the museum of lugs, on your right, are the Confente style of lugs - this is a classic Italian style and very common. It was created in the late 1970s.
In America, Henry James was the main supplier of quality cast lugs and the majority of frame builders used him as a supplier. As a result, many American frames looked similar.
One of the most ornate examples of lugs are those of Hetchins. These lugs had so many frills that some of them has steel extensions. in places such as the fork blades, these add-ons increased strength.

Chromed lugs, like those on the Hetchins, require that the entire frame be dipped in a chrome bath. To get the glossy finish of chrome, the bike is first nickel plated to get a smooth finish. The chrome layer is measured in millionths of an inch on this type of 'decorative' plating.