What shape is typically used for the topmark of Special Marks?

The topmark used for Special Marks in the IALA Maritime Buoyage System is a single yellow 'X'. This distinctive shape helps mariners to easily identify Special Marks, which are used to indicate features such as areas to be avoided, or to signify special zones or conditions. The yellow color is also important as it distinguishes these marks from others, as navigational aids come in various colors to convey specific meanings. The use of an 'X' as a topmark is particularly clear and recognizable, ensuring that it stands out against the backdrop of the sea and sky, enhancing navigational safety for vessels in the vicinity. This particular marking system promotes uniformity and understanding among mariners, as it indicates messages related to specific maritime conditions or regulations that may not be covered by more conventional buoyage marks.