1. The SEG standard for causal seismic data specifies that the onset of a compression from an explosive source is represented by a negative number, that is, by a downward deflection when displayed graphically; see Figure P-6. This standard is historically based; early refraction first arrivals broke downward. A reflection indicating an increase in acoustic impedance or a positive reflection coefficient also begins with a downward deflection. 2. For a zero-phase wavelet, a positive reflection coefficient is represented by a central peak, normally plotted black on a variable area or variable density display; see Figure P-6. This convention is called positive standard polarity and the reverse convention is negative standard polarity or reverse polarity. In the North Sea and some other areas the convention for zero-phase wavelet is reversed. Polarity standards are not specified for wavelets other than minimum-phase or zero-phase ones. See also dual polarity displays.