Slip Stitch in machine knitting refers to the process of missing stitches, allowing the working yarn to travel across the purl side of the knitting. These "floats" can be decorative when the purl side is used as the public side of the knitting.
The same process can be used to create Multi-color slip. Needles are missed (slipped) and colors are changed creating colorful, graphic designs.

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Slip classroom: