The wafer type has no bolt holes; the flange bolts pass around the outside of the valve. The lug type hae projections (lugs) that are tapped for the standard size of flange bolts. The flanges can then be cap screwed separately from both sides of the valve. Later, the piping can be removed from one side while the valve remains attached to the other.

Wafer style valves are designed to fit between 2 mating flanges & are held in place by friction from the bolts joining the flanges & compressing the valve between them. They may have one or more lugs on the outside of the valve body that are drilled or slotted so that flange mounting bolts will go through them, helping hold the valve in place during mounting & centring it on the pipe bore. This type of valve can often be rotated a few degrees after it is in place & before the flanges are tightened, until the valve stem hits a flange bolt.

Lug style valves have a row of bolt holes around the outside of the valve body(usually these are lugs off the valve body, hence the name- they could just make the valve body a bigger circle if they wanted to) thatare drilled to match up to the appropriate bolt pattern for the valve size & spec.These lugs are often tapped, & are structurally appropriate for mounting the valve directly to a flange without being compressed by a backup flange. They are located on the pipe bore by the bolt pattern and held directly by the bolts. Untapped lug style valves can also be used in place of a wafer style valve in a pipe run, simply by sliding the flange bolts all the way through the lugs & the mating flange.