One dozen normally represents 12, but a baker's dozen is 13 (one greater than a dozen).  This measurement originated as a surplus number, known as "the inbread," which was the direct result of bakers in England being careful to ensure compliance with the Assize of Bread and Ale law (first introduced into law during the reign of King Henry II {1154-1189 CE}, then again in 1266 CE by King Henry III) and selling bread that's not underweight; patrons who bought a dozen loaves of bread would get a 13th loaf, and patrons who bought smaller quantities would also get a portion of a 13th loaf, for free.

Perfect Baker's dozen

A number, such as 1,081,080, is classified as a "perfect baker's dozen" because it is a multiple of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, and 13, which makes it convenient to divide equally among any number of guests at just about any small social event such a family dinner (these small events typically don't exceed a dozen participants, and any number beyond 13 is not generally considered to be "small").

Although other numbers that aren't multiples of all these numbers might be referred to fractionally, for example a "half-perfect baker's dozen" or a "quarter-perfect baker's dozen," etc., these aren't exact enough to be useful in a practical way, and may only be convenient in stories, songs, and other types of folklore.