The trapezoidal groove represents a special design of the installation space for O-rings as a flange seal.
The groove flanks form a trapezoid with the groove base. The usual angle for aligning the groove flanks is approx. 25 degrees. The width of the groove is measured at the top edges of the groove and specified so that the groove holds an inserted O-ring.
Due to the special geometry, the production of this groove is very complex. The trapezoidal groove is also known as a dovetail groove and also exists as a half trapezoidal groove.