Bad:

a | b | c | a | b | c |

a | b | c | a | b | c |

a | b | c | a | b | c |

a | b | c | a | b | c |

a | b | c | a | b | c |

a | b | c | a | b | c |

a | b | c | a | b | c |

a | b | c | a | b | c |

a | b | c | a | b | c |

Better:

a | b | c | a | b | c |

a | b | c | a | b | c |

a | b | c | a | b | c |

a | b | c | a | b | c |

a | b | c | a | b | c |

a | b | c | a | b | c |

a | b | c | a | b | c |

a | b | c | a | b | c |

a | b | c | a | b | c |

Edit: removed flawed analysis. It's interesting to think about this with respect to bits of information conveyed to support quickly scanning a row horizontally.