Dear Editor, Recently Conca et al. [4] introduced the Inverse Knot for arthroscopic application in this Journal. Because both the first and the second throw of the knot may be tied either ‘‘over’’ or ‘‘under’’, these authors actually introduced four knots. Each of these four knots provides an instructive example of how seemingly different knots may turn out to be similar or, even, identical after they have been capsized or flipped. Capsizing refers to changing or perverting the knot’s configuration under asymmetrical stress, e.g. by putting more tension on its one end, than on the other [1, 7]. Conca et al. [4] illustrated how the knots they tied extracorporeally were immediately capsized into their sliding geometry to be advanced intracorporeally through the arthroscope (Fig. 1a, b). Thus, the sliding geometry of the over-andunder version of the Inverse Knot actually represents a capsized Surgeon’s Thief Knot (Fig. 1c). On first sight, a Thief Knot appears to be a Square Knot but there is a scarcely evident and essential difference, as it does not consist of two Overhand Knots [1]. Legend has it that sailors tie clothes bags and bread bags with this knot and that thieves always retie them with Square Knots and, hence, are detected. The Surgeon’s Knot is a Reversed Overhand Knot on top of a Double Overhand Knot [1, 6], and a Surgeon’s Thief Knot differs from the Surgeon’s Knot in the same way as the Thief Knot differs from the Square Knot. This Surgeon’s Thief Knot may alternatively be capsized by pulling the other end. It will, then, become a Preformed Loop that was previously advocated for endoscopic use by Croce et al. [5] (Fig. 1d). Likewise, the under-and-under version of the Inverse Knot is a capsized Surgeon’s Whatknot (the Whatknot being a granny-wise tied Thief Knot) that can be alternatively capsized to become an Adjustable Jam Hitch (Ashley’s knot #1994) [1] (Fig. 2a–d), whereas the under-and-over version is a capsized Reversed Surgeon’s Thief Knot that can also be capsized into the fisherman’s Improved Clinch Knot [2, 8] (Fig. 3a–d). The Reversed Surgeon’s Knot is a Reversed Double Overhand Knot on top of an Overhand Knot and [6], again, the Reversed Surgeon’s Thief Knot differs from the Reversed Surgeon’s Knot like the Thief Knot differs from the Square Knot. Finally, the over-and-over version of the Inversed Knot is a capsized Reversed Surgeon’s Whatknot that may be alternatively capsized to become a Magnus or Roller Hitch (Ashley’s knot #1230) [1] (Fig. 4a–d). This possibility to capsize a flat knot either way indicates that its one sliding geometry may also immediately be flipped to its alternative sliding geometry. Flipping the knot means to interchange the standing part and working end of the suture by pulling more on the latter than on the first, thereby relocating the knot from one strand of the suture, to the other [3]. The Inverse Knot flips in a special fashion as its geometry actually changes when it is flipped. In conclusion, the work of Conca et al. [4] is laudable for presenting an original knot along with a simple technique to extracorporeally tie it. The flipped modifications of all four versions of this knot have previously been recognized and named [1, 2, 5, 8], and their value during endoscopic surgery has been recorded [5]. Understanding how knots may be converted and why seemingly different knots actually represent capsized or flipped modifications of each other provides surgeons with an insight in the J. J. Hage (&) Section of Surgical Oncological Disciplines, Netherlands Cancer Institute, Antoni van Leeuwenhoek Hospital, Plesmanlaan 121, 1066 CX Amsterdam, The Netherlands e-mail: j.jorishage@inter.nl.net