The particles composed of charm c and anti-charm c quarks, like J/ψ meson, have no charm property (quantum number). Particle physicists like to call this kind of feature hidden-charm. On the other hand, charmed particles, like D mesons, composed of charmed quark and no anti-charm quark, or vice versa, have charm property (quantum number) and physicists call them open-charm particles. The Tcccc tetraquark is a hidden charm particle as are all the other tetraquarks observed up to now. The newly discovered tetraquarks, cdus, could be interpreted as the first discovery of an open-charm tetraquark.