On a typical butt weld, the two parts to be welded are pressed close together and welded. If the parts are thin enough, the weld can go deep enough to weld the entire weld interface (a.k.a. CJP- Complete Joint Penetration), as would be required if the weld is expected to transfer load from one side to the other. "Thin" is a relative term since processes like Electron Beam Welding can butt weld fairly thick sections, of the order of a few inches. However, arc welding processes are limited to less than 0.25" (~6 mm) before they need special edge preparation to allow them to make a CJP weld.

In order to gain access for the welding electrode to the whole depth of the weld, the parts have to be machined to have a wider gap at the top, as is shown in the schematic below. The drawing shows both parts have been machined to form a V-groove between the parts with an included angle of 60 degrees. Now the electrode can have access to the bottom of the weld from where the process will start. If the parts are quite thick that a single pass is not enough to fill up the space, the operator will lay down multiple passes to fill up the groove. Multiple passes themselves posses huge challenges including cleaning, pre-heating, residual stress, and flaw detection. The first pass at the bottom of the groove is the trickiest since it has to bridge the gap and produce good fusion with the weld faces. If the gap is too big, a backing plate can be used and can be left in place after welding. When a backing plate is not used, the first pass may have to be partially gouged out and re-welded from the other side to remove any defects.

As plate thickness increases, the groove volume becomes bigger (to maintain the same groove angle for electrode access) and at some point it is better to make V-grooves from both sides, called the Double-V groove joint. The Double-V requires less filler metal volume and, with proper management of welding pass sequence, can help in balancing stresses induced by welding. In addition to the "V" shape, welding designers have come up with other shapes for the groove including "U", "J", and bevel (half-V; only one part is machined, the other has a flat face), with their corresponding twins: Double-U, Double-J, and Double-Bevel. For thicker parts, you have many choices in terms of groove shape, size, number of welding passes, welding sequence, gouging method, etc.., but one thing is for sure - you gotta groovy kind of weld.