Its getting even more complicated nowadays as tubeless tyres are being introduced (like car tyres), these need another type of rim, but some top-end wheels are now available with rims that can take both tubeless and clinchers. Tubulars cannot fit onto a clincher type rim and viceversa. A tubular rim is smooth and hollowed on the tyre-side to allow the tubular tyre to be glued to it, rather like a hollow fat rounded, filled in, C in section.