If the feature is present, we mark it with a plus sign (+) and if it’s not present, we use a minus sign (−). Thus /p/ can be characterized as [− voice, + bilabial, + stop] and /k/ as [− voice, + velar, + stop]. Because these two sounds share some features (i.e. both are voiceless stops),
Yule, George. The Study of Language (p. 41). Cambridge University Press. Kindle Edition.