The needle is inserted at the top of the stitch that is being made and then it is made to emerge next to the lower limit of the stitch. The needle is brought out over the working thread, so forming a long straight stitch with a looped edge on the lower line. Its form suggests the letter L.
This method of making a stitch is related to that used for feather stitches. The looped edge is useful as an edging, as the tight row of loops prevents the material from fraying when it is afterwards cut away, as in cutwork and Richelieu work.
The buttonhole stitch is also known as the button stitch or the close stitch.
Source: THOMAS, Mary (1934). Mary Thomas's Dictionary of Embroidery Stitches, London: Hodder and Stoughton, p. 21.