 Audio transformers for vacuum tube equipment were coded blue for the finishing lead of the primary, red for the B+ lead of the primary, brown for a primary center tap, green for the finishing lead of the secondary, black for grid lead of the secondary, and yellow for a tapped secondary.Each lead had a different color since relative polarity or phase was more important for these transformers.Intermediate-frequency tuned transformers were coded blue and red for the primary and green and black for the secondary.