Diagnostic diagrams: [OIII]λλ5007/Hβ vs. [NII]λλ6548, 6584/Hα top, and [OIII]λλ5007/Hβ vs. [SII]λλ6717, 6731/Hα bottom. Grids represent the prediction from the shock models using libraries from Allen et al. (2008). Numbers over the lines indicate the value of the shock velocity represented by red lines, while the blue lines are models with equal magnetic field. Crosses are the pixels from NGC 6888 maps belonging to the Zone B. Only data from Zone B are represented well in both diagrams by a model with shock and precursor, velocities from 250 to 400 km s^-1, n = 1000 cm^-3, and twice solar abundance.