▁and ▁thus ▁' making ▁the ▁book '. ▁ ▁M aking ▁a ▁' book ' ▁( and ▁the ▁notion ▁of ▁over round ) ▁A ▁book maker ▁st riv es ▁to ▁accept ▁bet s ▁on ▁the ▁outcome ▁of ▁an ▁event ▁in ▁the ▁right ▁proport ions ▁in ▁order ▁to ▁make ▁a ▁profit ▁regardless ▁of ▁which ▁outcome ▁prev ails . ▁See ▁Dutch ▁book ▁and ▁coh er ence ▁( ph il osoph ical ▁g amb ling ▁strategy ). ▁This ▁is ▁achieved ▁primarily ▁by ▁adjust ing ▁what ▁are ▁determined ▁to ▁be ▁the ▁true ▁odd s ▁of ▁the ▁various ▁out comes ▁of ▁an ▁event ▁in ▁a ▁down ward ▁fashion ▁( i . e . ▁the ▁book maker ▁will ▁pay ▁out ▁using ▁his ▁actual ▁odd s , ▁an ▁amount ▁which ▁is ▁less ▁than ▁the ▁true ▁odd s ▁would ▁have ▁paid , ▁thus ▁ens uring ▁a ▁profit ). ▁ ▁The ▁odd s ▁quoted ▁for ▁a ▁particular ▁event ▁may ▁be ▁fixed ▁ ▁but ▁are ▁more ▁likely ▁to ▁fl uct uate ▁in ▁order ▁to ▁take ▁account ▁of ▁the ▁size ▁of ▁w agers ▁placed ▁by ▁the ▁bet t ors ▁in ▁the ▁run - up ▁to ▁the ▁actual ▁event ▁( e . g . ▁a ▁horse ▁race ). ▁This ▁article ▁explains ▁the ▁mathematics ▁of ▁making ▁a ▁book ▁in ▁the ▁( sim pler ) ▁case ▁of ▁the ▁former ▁event . ▁For ▁the ▁second ▁method , ▁see ▁Par im ut uel ▁bet ting . ▁ ▁It ▁is ▁important ▁to ▁understand ▁the ▁relationship ▁between ▁fraction al ▁and ▁decimal ▁odd s . ▁Fra ction al ▁odd s ▁are ▁those ▁written