On Wed, Jul 21, 2004 at 11:09:00PM +0200, Andreas Johansson wrote:

> "In deference to the requirements of those working on French, SAMPA
> has defined a special role for the forward slash, / (ASCII 47), namely
> as a marker of certain vowel archiphonemes or indeterminacies, e.g.
> maison /mE/zO~/. It is of course also widely used as a delimiter of
> phonemic transcriptions."

Okay, I had to look that one up.  An "archiphoneme" is the result of the
partial neutralization (that is, merger) of two other phonemes. If
there's a complete merger, you only have one phoneme, so there's no
problem; but often you find partial mergers, where two normally-distinct
phonemes are neutralized only in some set phonetic contexts.  Because of
the neutralization, it is misleading to indicate in the notation that
a specific one of the merged phonemes is present.

The example given in my dictionary is the final sound of the German
words |Rat| and |Rad|.  Both are pronounced the same way, even though
/t/ and /d/ are separate phonemes in other environments.   So instead
of writing /Rat/ or /Rad/, either of which implies a distinction which
is not made, you can use archiphonetic notation, which normally involves
slashes around the symbol for one of the phonemes involved in the
merger: /Ra/t//

Usually archiphonetic notation involves capitalization as well as the
slash (e.g. /Ra/T//), but not in [X-]SAMPA, where capitalizing a symbol
changes the meaning (/T/ is IPA theta, for instance).  Judging by the
example Andreas quoted, the X-SAMPA usage has it as a trailing modifier
rather than a bracketing delimiter, too, so my example would be
written /Rat//.

In the specific case of |maison| I gather that the distinction which has
been neutralized is between /e/ and /E/.