Lap joint flanges feature a flat face and are always used in conjunction with a stub end.
Lap joint flanges resemble, in shape, slip-on flanges except for the radius at the crossing of the flange face and the bore to accommodate the flanged portion of the stub end.
A lap joint flange slips over the pipe and seats on the back of the stub end and the two are kept together by the pressure of the bolts.
The use of lap joint flanges in combination with stub ends is a cost-effective solution for stainless steel or nickel alloy pipelines, as the material of the lap joint flange can be of a lower grade (generally carbon steel) than the material of the stub end (which has to match the pipe grade, as in contact with the conveyed fluid).