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1.
*In relation to the Japanese Visual Kei movement*
A decagram is a ten-pointed star similar to a pentagram. It can be drawn with one simultnious line, like a pentagram, or with two inter-locking pentagons.
It is the OFFICIAL symbol of the band Moi Dix Mois lead by guitarist Mana (who is something of a legend in the Visual Kei world)
'Dix' meaning 'ten' in french is where the band's theme of Ten comes from. Fans say Dix in conversation about the band and some will bear pendents of the decagram.
A lyric in a song by Moi Dix Mois is - "The decagram, our symbol of pride, will show us the way to the sacred world,"
by Lady Tussuad October 06, 2010
 
2.
Similar to hexagram, or pentagram, only with ten sides. Many bands use this symbol. Some good, some bad.
Slipknot, a shitty nu-metal band, often uses this logo, and so many believe that it is their trademarked symbol.

"dude, the knot are SO tight! I'm getting their decagram tattooed on my arm!" "you do realize that isn't Slipknot's 'symbol', don't you?" "shut up, the knot are omnipotent!" "lol, trendwhore"
by count funkulator November 07, 2007