In 1921 a method was formulated for calculating the articulation—the percent of speech sounds interpreted correctly during a telephone conversation—from the physical characteristics of the telephone system and the noise environment of the listener. This paper describes this empirical theory as recently revised to correspond to modern techniques. A quantityAis calculated which is called the articulation index and which is related to the various kinds of measured values of articulation. In particular, ifsis the percent of the fundamental sounds correctly perceived by the listenerA= −plog (l‐s), wherepis a factor depending upon the skill of the crew making the articulation tests. For a well‐practiced crew the value ofpis between 0.5 and 0.6. The value ofAis calculated by the equationA = F⋅V⋅E, whereFis the maximum value ofA, andVa factor which is zero at threshold intensity of the speech received by the listener and grows as the received speech intensity increases reaching unity for intensities between 60 and 70 db above threshold. The factorEis unity below this intensity but decreases from unity to 0.85 as the intensity levels increase to 60 db above these levels. The factorFis calculated by the equationF = ∫0∞D⋅Wdf, whereDis a function of the frequencyf, andWis a function of the relative response of the system. The functionsDandWhave been determined directly from articulation data. The factorsV(x) andE(x) are determined directly from articulation data. The value ofxfor systems in a quiet place is equal to a weighted average of the response of the system. When a noise is present which produces a maskingMat each frequency at the listener's ear, the same functions are used for determiningFandV, but the responseRat each frequency is replaced byR − M. However, for this case thexinE(x) is increased by an amount Δ, a quantity calculated directly from the maskingMof the noise and the intensity of the received speech.