Each type of weld has its own basic symbol, which is typically placed near the center of the reference line (and above or below it, depending on which side of the joint it's on). The symbol is a small drawing that can usually be interpreted as a simplified cross-section of the weld. In the descriptions below, the symbol is shown in both its arrow-side and other-side positions.

The tail of the weld symbol is the place for supplementary information on the weld. It may contain a reference to the welding process, the electrode, a detail drawing, any information that aids in the making of the weld that does not have its own special place on the symbol.

The open circle at the arrow/reference line junction is present if the weld is to go all around the joint, as in the example below.

The flag growing out of the junction of the reference line and the arrow is present if the weld is to be made in the field during erection of the structure. A weld symbol without a flag indicates that the weld is to be made in the shop. In older drawings, a field weld may be denoted by a filled black circle at the junction between the arrow and the reference line.

The weld symbol distinguishes between the two sides of a joint by using the arrow and the spaces above and below the reference line. The side of the joint to which the arrow points is known ( rather prosaically ) as the arrow side , and its weld is made according to the instructions given below the reference line. The other side of the joint is known (even more prosaically) as the other side , and its weld is made according to the instructions given above the reference line. The below=arrow and above=other rules apply regardless of the arrow's direction.

Quite often, there are two sides to the joint to which the arrow points, and therefore two potential places for a weld. For example, when two steel plates are joined together into a T shape, welding may be done on either side of the stem of the T.

The horizontal line--called the reference line --is the anchor to which all the other welding symbols are tied. The instructions for making the weld are strung along the reference line. An arrow connects the reference line to the joint that is to be welded. In the example above, the arrow is shown growing out of the right end of the reference line and heading down and to the right, but many other combinations are allowed.

Notice that the spacing, or pitch , is not the clear space between the welds, but the center-to-center ( or end-to-end ) distance.

For intermittent welds, the length of each portion of the weld and the spacing of the welds are separated by a dash ( length first, spacing second ) and placed to the right of the fillet weld symbol.

If no length is given, then the weld is to be placed between specified dimension lines ( if given ) or between those points where an abrupt change in the weld direction would occur ( like at the end of the plates in the example above ).

The length of the weld is given to the right of the symbol.

The perpendicular leg of the triangle is always drawn on the left side of the symbol, regardless of the orientation of the weld itself. The leg size is written to the left of the weld symbol. If the two legs of the weld are to be the same size, only one dimension is given; if the weld is to have unequal legs ( much less common than the equal-legged weld ), both dimensions are given and there is an indication on the drawing as to which leg is longer.

The fillet weld ( pronounced "FILL-it," not "fil-LAY" ) is used to make lap joints, corner joints, and T joints. As its symbol suggests, the fillet weld is roughly triangular in cross-section, although its shape is not always a right triangle or an isosceles triangle. Weld metal is deposited in a corner formed by the fit-up of the two members and penetrates and fuses with the base metal to form the joint. ( Note: for the sake of graphical clarity, the drawings below do not show the penetration of the weld metal. Recognize, however, that the degree of penetration is important in determining the quality of the weld. )