Why did the ca. 1797 closet not use mortises like the ca. 1764 closet?  Nail holes on the floor indicated that a spline, or long wooden strip, had been installed on the floor.  To install the studs rabbets, or U-shaped grooves the width of the splines, were cut into the bottom of the studs.  Next the carpenters in ca. 1797 simply set the bottom of the stud onto the spline, nailed into the spline and then secured the upper end with nails anchored into the ceiling joists.  The advantage that this system had over the ca. 1764 mortises was that it was quicker and required less skill, especially if the stud was warped or not perfectly straight.