[gregwebs]

My understanding from 5 years ago is that in MRI the leading bit is reserved to indicate whether something is an integer. This of course reduces the size of numbers that fit into a machine word but it seems like a pretty good tradeoff.

[icebraining]

Yeah, apparently they're called tagged pointers. The More You Know.

EDIT: And here's Guido explaining why he didn't want that in Python: https://mail.python.org/pipermail/python-dev/2004-July/04614...

[lispm]

Most Lisp systems use tags. Several data types will have tags included in the data word, like fixnums, characters. Other data types have an extra word.