דברים כא ,יז : לָתֶת לוֹ פ ִי שְׁנַּיִם(To give to him a double portion)
Baal HaTurim states that each letter of the letters in the word בכור alludes to the concept of doubling. The letter ב (value 2) is double the value of א, the preceding letter. Also, the letter כ (value 20) is double of the value of י (10), its preceding letter. Plus the letter ר (value 200) is double its preceding letter (ק 100). Even the letter Vav contains a double ו"ו (the name of the letter).