To switch to another family one may use \rm,
\sf or \tt denoting the `cmroman', `cmsansserif' or
`cmtypewriter' family, respectively. The new font is selected
without changing the current series, shape, and size. E.g., if the current font is cmbx10 (that is family `computer modern roman', series `bold extended',
shape `normal', and size `10pt') then \sf will
change to cmssbx10 (that is family `computer modern
sansserif', series `bold extended', shape `normal', and
size `10pt'). Using, e.g., \tt afterward will
produce a warning and switch to cmtt10 because the `computer
modern typewriter' family does not contain a `bold extended'
series; therefore the default (`medium') series is tried.