"Each new House elects a Speaker by roll call vote when it first convenes. Customarily, the conference of each major party nominates a candidate whose name is placed in nomination. Members normally vote for the candidate of their own party conference, but may vote for any individual, whether nominated or not. To be elected, a candidate must receive an absolute majority of all the votes cast for individuals. This number may be less than a majority (now 218) of the full membership of the House, because of vacancies, absentees, or Members voting 'present.' This report provides data on elections of the Speaker in each Congress since 1913, when the House first reached its present size of 435 Members. During that period (63rd through 113th Congresses), a Speaker was elected four times with the votes of less than a majority of the full membership."
CRS Report for Congress, RL30857
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html