Butted aluminum tubes—where the wall thickness of the middle sections are made to be thinner than the end sections—are used by some manufacturers for weight savings. Non-round tubes are used for a variety of reasons, including stiffness, aerodynamics, and marketing. Various shapes focus on one or another of these goals, and seldom accomplish all. Titanium is perhaps the most exotic and expensive metal commonly used for bicycle frame tubes. It combines many desirable characteristics, including a high strength to weight ratio and excellent corrosion resistance. Reasonable stiffness (roughly half that of steel) allows for many titanium frames to be constructed with "standard" tube sizes comparable to a traditional steel frame, although larger diameter tubing is becoming more common for more stiffness. Titanium is more difficult to machine than steel or aluminum, which sometimes limits its uses and also raises the effort (and cost) associated with this type of construction. As titanium frames are usually more expensive than similar steel or aluminium alloy frames, the cost puts them out of reach for most cyclists. Titanium frames typically use titanium alloys and tubes that were originally developed for the aerospace industry. The most commonly used alloy on titanium bicycle frames is 3AL-2.5V (3.5% Aluminum and 2.5% Vanadium). 6AL-4V (6% Aluminum and 4% Vanadium) is also used, but it is more difficult to weld, make tubes of, and machine. Often, the tubes are of 3AL-2.5V while dropouts and other peripheral sections are made of 6AL-4V. Experimental frames have been made with commercially pure (CP, i.e.:unalloyed) titanium, but these proved less durable for the active riding intended for frames of this cost level. Extensive butting is also used to create low weight tubes with acceptable stiffness. The early versions of the Fat Chance Titanium (1992 and 93 versions) had tubes of different diameters welded together to create a stiffer bottom bracket area. The 1994 version had externally butted bottom tubes. Frame tubes are almost always joined by Gas Tungsten Arc Welding (GTAW or TIG) welding, although vacuum brazing has been used on early frames. Some earlier titanium frames were made with titanium tubes bonded to aluminum lugs, as e.g. the Miyata Elevation 8000 and the Raleigh Technium Titanium. Carbon fiber composite is an increasingly popular non-metallic material commonly used for bicycle frames. Although expensive, it is light-weight, corrosion-resistant and strong, and can be formed into almost any shape desired.