The distortion of the winding curve reflects the reality of signal transmission along a line where the phase change to the frequency is not linear, that is, the phase tends to change when the signal is transmitted through the communication connection. The phase shift is calculated by deflecting the phase by the signal frequency at any point along the line. The slope of the phase against the frequency is called the offset of the envelope curve. Delay distortions create problems because two different frequencies (which display a “1” or a bit 0) interfere with each other on the receiving modem, causing a potential error called inter-symbol interference. A modified version of the Baudot Code has been adopted by ITU as a standard for telegraph communication. It uses two “Shift” characters for letters and numbers and was the precursor to modern ASCII and EBCDIC codes.