text: Another useful way of determining how many protons a carbon in a molecule is bonded to is to use an attached proton test (APT), which distinguishes between carbon atoms with even or odd number of attached hydrogens. A proper spin-echo sequence is able to distinguish between S, IS and IS, IS spin systems: the first will appear as positive peaks in the spectrum, while the latter as negative peaks (pointing downwards), while retaining relative simplicity in the spectrum since it is still broadband proton decoupled.
  Even though this technique does not distinguish fully between CH groups, it is so easy and reliable that it is frequently employed as a first attempt to assign peaks in the spectrum and elucidate the structure. Additionally, signals from quaternary carbons and other carbons with no attached protons are still detectable, so in many cases an additional conventional C spectrum is not required, which is an advantage over DEPT. It is, however, sometimes possible that a CH and CH signal have coincidentally equivalent chemical shifts resulting in annulment in the APT spectrum due to the opposite phases. For this reason the conventional C{H} spectrum or HSQC are occasionally also acquired.