Trent Alexander-Arnold wore the number 66 at Liverpool, but La Liga rules prevent him from doing so at Real Madrid.
Spanish top-flight clubs must name a 25-man 'A' squad at the start of each season, and every player included is obliged to wear a number from 1-25, with goalkeepers allocated the numbers one, 13, and 25.
Youth players not included in the initial 'A' squad can wear higher numbers.
At the time of Alexander-Arnold's signing, the only outfield numbers available between one and 25 were 12 and 18, and the England international opted for the former.
Real's number 12 was previously worn by France midfielder Eduardo Camavinga, but he switched to number six a year ago after the departure of Nacho Fernandez.
Prior to that, it was worn for 16 years by Brazilian left-back Marcelo, who won 25 major trophies during his time with the club.
Former Bolton defender Ivan Campo also wore that number during his four-year stint at the Bernabeu.