The Canadian Corps' divisional artillery formations, totalling eight field artillery brigades and two heavy artillery groups, were insufficient for the task at hand and were consequently reinforced with outside formations. Four heavy artillery groups, nine field artillery brigades, three divisional artillery groups and the artillery complement of the British 5th Division was attached to the Canadian Corps. In addition, ten heavy artillery groups of the flanking I and XVII Corps were assigned tasks in support of the Canadian Corps. The artillery batteries of I Corps were particularly important because they enfiladed German gun positions behind Vimy Ridge. In total, the British made available to the Canadian Corps twenty-four brigade artillery groups consisting of four hundred and eighty 18 pounder field guns, one hundred thirty-eight 4.5 inch howitzers, ninety-six 2 inch trench mortars, twenty-four 9.45 inch mortars, supported by 245 corps-level siege guns and heavy mortars. This firepower gave a density of one heavy gun for every and one field gun for every of Canadian Corps frontage, representing a considerable average increase, including three times the heavy guns, over the distribution of artillery at the Battle of the Somme a year earlier.