Lurking P waves - The Bix Rule
Whenever you are looking at ECG tracings of supraventricular tachycardia, in which visible P waves are situated midway between QRS complexes, there is a probability that there are also P waves lurking within the QRS complexes. Thus, atrial tachycardia or atrial flutter with 2:1 AV conduction are possible and need to be adressed for differential diagnosis.
The term "Bix Rule" was named after viennese cardiologist Harold Bix. To remember Bix Rule in short: "QRS-P-QRS"