Golf shafts come in different shaft flexes, weights, lengths and materials so the club maker can build a club that fits each golfer's swing. Steel shafts come in two tip types, taper and parallel. The tip is the part of the shaft that fits into the hosel of the clubhead. The difference between the two types of shaft is the tip diameter and shaft weight.
Just as its name implies, a taper-tip shaft tapers, getting smaller where it fits into the hosel. The standard taper-tip shaft for irons is .355 inch at the end and requires a clubhead with a hosel the same size. Taper-tip shafts are manufactured to the proper length for each club. For example, a 9-iron shaft would not be a proper fit for an 8-iron. Taper-tip shafts are also manufactured with constant weights, meaning the 3-iron shaft weighs the same as the 9-iron shaft.
With a parallel-tip shaft, the entire tip section--the portion below the last step--is a constant diameter. The standard size is .370 inch for irons and would require a clubhead with a .370-diameter hosel hole. Parallel-tip shafts are the same, within the model type, for each iron but are trimmed to fit the length for each club. As a result, parallel-tip shafts are heavier in the longer irons and weigh less in the shorter irons.
The taper tip was the standard until the 1970s, when the parallel tip was introduced. The parallel-tip shaft allowed club makers to control inventory because they could stock just one shaft length for each model of woods and irons, instead of stocking a different length shaft for each club. With parallel-tip shafts, the manufacturers trim the tip for flex and trim the butt (top) of the shaft for length.
To adjust shaft flex with parallel-tip shafts, you change the amount you trim. To stiffen the shaft, trim more off the tip end; to weaken the flex, trim less off the tip and make the club longer. To adjust the flex with taper-tip shafts, however, you have to use a different shaft. For a stiffer flex, use the next shorter shaft (put a 5-iron shaft in a 4-iron clubhead). To soften the flex, step up one shaft length (put a 3-iron shaft in a 4-iron clubhead).
Tour professionals and traditionalists prefer the taper-tip shaft because of the constant weight, but the average golfer will notice little difference. According to True Temper, taper- and parallel-tip shafts play the same, and blind testing with identical iron heads showed the testers noticed no difference in the clubs.