They are cutting mortises in those girts.
"The Doers"
William John Hopkins
The mortises and tenons were found to be as sound and clean as on the day when they were fitted by the sixteenth-century carpenters.
"Vanishing England"
P. H. Ditchfield
They pass through two horizontal beams, which string them, as it were, together, and while the mortises in the beams are so loose, as to let the pestles work vertically, it restrains them to that motion.
"Memoir, Correspondence, And Miscellanies, From The Papers Of Thomas Jefferson"
Thomas Jefferson