>On their banner was a giant in shattered chains that told him that these were Umber men, down from the northlands beyond the Last River.
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>Along the walls the banners hung: the horseheads of the Ryswells in gold, brown, grey, and black; the roaring giant of House Umber; the stone hand of House Flint of Flint's Finger; the moose of Hornwood and the merman of Manderly; Cerwyn's black battle-axe and the Tallhart pines.
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>The badges on their jerkins were smaller versions of the sigil sewn on their master's surcoat; a black pitchfork on a golden bar sinister, upon a russet field. Arya had thought of revealing herself to the first outriders they encountered, but she had always pictured grey-cloaked men with the direwolf on their breasts. She might have risked it even if they'd worn the Umber giant or the Glover fist, but she did not know this pitchfork knight or whom he served.
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>"Ramsay Bolton, Lord of Winterfell, he signs himself. But there are other names as well." Lady Dustin, Lady Cerwyn, and four Ryswells had appended their own signatures beneath his. Beside them was drawn a crude giant, the mark of some Umber.
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Sigil depiction and recognition are obviously of great importance in Westeros. The majority of them are easy enough for highborn and low alike to recognize, or even replicate for a badge or marker - a horizontal grey smear with the right number of limbs and a vaguely canine head shape on a field would white would be universally recognized as the arms of House Stark, as would a golden four-legged shape in a vaguely rampant pose on a field of red for Lannister, a red three-headed shape on a field of black for Targaryen, and so on.
Overall the designs are simple enough: axes, keys, pines, animals with one or two distinguishable features, etc. Even the notable houses which depict a human form have something remarkable and easy to reproduce: draw your human with a plow in front and that's Darry, put a bow in his hand and it's the huntsman of Tarly, draw him in pink - maybe even add some lines to imply bare musculature - and it's the flayed man of Bolton.
So what visual shorthand or symbol marks a human shape as being "a giant"? It's not solely the Umber colors which signify it, as the Pink Letter's signatures show that even a sketch can be recognized as "a giant". Could it be proportionally-larger limbs or extremities? The roaring? Wooliness? Certainly, it's not an accompanying human drawn to scale.
Is it the chains, as they are depicted on the Umber arms, and if so, why? Are giants depicted or understood to have been "in chains", outside of the Umber sigil - possibly enslaved during the raising of The Wall? Or does Umber history somehow involve a giant breaking out of manacles, and this local symbolism has extended to the continent to not just mean "Umber", but also, "giant"? Notably, the HBO adaptation foregoes pure accuracy for heraldry, and the Umbers in the show blazon a pair of crossed chains.