Without an actual comb one can look at the phase vs frequency. Without a carrier–envelope offset all frequencies are cosines. This means that all frequencies have the phase zero. The time origin is arbitrary. If a pulse comes at later times, the phase increases linearly with frequency, but still the zero-frequency phase is zero. This phase at zero frequency is the carrier–envelope offset. The second harmonic not only has twice the frequency, but also twice the phase. Thus for a pulse with zero offset the second harmonic of the low-frequency tail is in phase with the fundamental of the high-frequency tail, and otherwise it is not. Spectral phase interferometry for direct electric-field reconstruction (SPIDER) measures how the phase increases with frequency, but it cannot determine the offset, so the name “electric field reconstruction” is a bit misleading.