Microtubules
Microtubules are the largest of the cytoskeletons measuring 23 nm in diameter. Generally comprising 13 protofilaments that are polymers of alpha and beta tubulin. Their vital function is to bind GTP for polymerization. Microtubules are commonly organized by the centrosome forming centrioles. In specific situations, two additional microtubules are utilized and oriented around in order to form cilia and flagella. This formation is commonly referred to as a "9+2" arrangement due to the nine doublets of microtubules arranged around the two extra microtubules, each doublet is connected to another by the protein dynein. Seeing as flagella and cilia are structural components of the cell and are maintained by microtubules they can be considered part of the cytoskeleton.