a) attribute cell (white)
b) palettes

Attribute Cells are evenly dimensioned subsections of an image which attach local constraints or properties to the pixels included in that section.

Historically, attribute cells exist due to memory limitations and specific memory layouts for different graphic chips.

In this arbitrary invented example, the attribute cells are 4×8 pixels, the pixels are stretched horizontally and each pixel refers to exactly one of four indices.

Furthermore, the properties for each attribute cell here are stored in four bits per cell which select one of four global palettes and there are two flip bits which specify whether the cell contents should be flipped across the vertical and/or the horizontal axis.

further reading:

• ZX Spectrum graphic modes
• C64 Graphic Modes on studiostyle.sk