Carbons couple with the hydrogen atoms that are directly attached to them.
Thus, a methyl group (-CH3) appears as a quartet
in the 13C-NMR spectra.
A methylene group with two attached hydrogens appears as a triplet, a methine group (-CH) appears as a doublet, and a quaternary carbon, with no hydrogens attached, appears as a singlet.
Carbon-13 will also couple with another directly bonded carbon-13 atom but the odds of this are very low (.01%) in naturally occurring samples and this generally is not seen. With this coupling to bound hydrogens, a 13C-NMR spectra can appear as a real jungle of lines.
To improve the spectra allowing easy counting of the number of carbons, the protons are often "decoupled" from the carbons by irradiating them at a frequency that excites them and interrupts their normal coupling. In the absence of coupling from the protons, the carbon resonances all appear as singlets and such a spectrum is said to be "Proton Decoupled".
Proton decoupled 13C-NMR is probably the most widely used technique because it clarifies the 13C-NMR spectrum making it easier to determine the number of carbon atoms. The student should be aware that the proton coupled experiment does exist and can provide useful information on how many hydrogen atoms are bound to a particular carbon.