If crossing-over occurs twice at the same time a middle piece of one chromosome is Intercalated between the ends of the other chromosome.
"Sex-linked Inheritance in Drosophila"
Thomas Hunt Morgan Calvin B. Bridges
In order to prevent that inconvenience, it is necessary to suppose that the Indians, whether they Intercalated the additional day at the end of the 18 months or after the five supplementary days, did not only give to it the same number and hieroglyphic as to the day immediately preceding, but also designated it by some peculiar sign or number, in order that it might not be confounded with any other.
"Incidents of Travel in Yucatan, Vol. I."
John L. Stephens
They considered those four years as Intercalated; therefore believed them to be unfortunate, and called them u yail Jaab, as they did the five supplementary days of the year, to which they likened them.
"Incidents of Travel in Yucatan, Vol. I."
John L. Stephens