Each sub-system is associated with a particular observing technique (or their combination) used to discover and study it, which hereafter is called type. In MSC it is coded as a sequence of letters. Their meaning is explained below.
Wide physical double stars are coded by the letter C (for C.P.M. - common proper motion pairs). It is followed by one or several small letters describing what criteria were used in testing the physical relation of the components:
.
, where 
is
the apparent separation of a binary and 
is the relative
motion of components, is on the average proportional to
,
where p is parallax and M is
mass sum in solar units (Russel & Moore 1940). Relative motion
was computed form the first and the last observations given in
WDS (admittedly a risky procedure because of the
insufficient accuracy of the observations).
This criterion has only a statistical sense, but it proved
to be very useful in rejecting optical companions.
When neither of these criteria is verified the system is designated by c. In some cases there is also a question mark which means a likely optical system or a contradiction between different criteria. Uncertain wide systems are included only if one of their components is at least triple.
When components are resolved and are separated by less than
3
the system is called "visual" and is designated by v.
Interferometrically resolved binaries
are also included in this category.
It is assumed on statistical grounds that all close
visual systems are physical. In some cases it can be proved in
the same way as for wider systems, e.g. by hypothetical
parallax.
Visual binaries with computed orbits are designated by V.
Binary stars discovered by lunar occultation technique are
coded by o.
When the binary nature is deduced by astrometric techniques (i.e. by deviations from linear motion on the sky or from the keplerian motion of components in a wider sub-system) the letter a is used, or A if an astrometric orbit is computed.
Binarity discovered from velocity variations or from composite spectral type is coded as s (or as s2 if double lines were noted). Spectroscopic binary stars with computed orbits have designations S1 (single-lined) or S2 (double-lined).
Eclipsing binary stars have code E if orbital period is known, code e otherwise. Code E* means a sub-system discovered by eclipse timing.
The traditional identification of visual components by upper-case letters A, B, etc. as given in WDS was kept in MSC, to avoid component confusion as much as possible. The additional visual components are designated by letters P, Q, etc. as usual. Spectroscopic sub-systems are identified by appending lower-case letters, e.g. Aa, Ba. A "component'' may turn into "sub-system'' if a new close component is discovered. However, the sub-systems already known would not be modified by such a discovery, only their level might change.
The masses of individual components or sub-systems in solar mass units are given in the MSC. Mass codes describe the method used for the estimation:
, mass of white dwarfs
is assumed to be 1 
.
, e.g. for interferometric
binaries.
The order in which the items are listed reflects the priority of different mass estimation procedures which are chosen automatically according to the available information and the type of system. Evidently, the quality of mass estimates given in MSC is very different, but the mass code indicates the quality without ambiguity.
The methods of distance estimation and the corresponding codes are listed below in order of preference:
is
assumed.
The decimal logarithm of orbital period in days is given
for each sub-system. For sub-systems with known orbits (types V, A, S1,
S2, E) it is the true orbital period, otherwise it is
estimated from component separation, distance to the system
and mass sum. It is assumed that the observed component
separation 
is equal to the semi-major axis of the
orbit a
(the statistical studies show that the average factor relating a
to 
is indeed close to unity). These estimates are useful for
statistical purposes and as an indication of the possibility of
orbit computation.
The meaning of the angular separation depends on
system type. For systems with known visual orbit it is equal to
a. For visual and C.P.M. systems 
is given (if it
changed with time, the first observation is usually given).
For occultation binaries the projected separation is given.
Finally, for spectroscopic and eclipsing systems a is
calculated by the third Kepler law. The position angles are also
given. They can be helpful for identification of wide components
but practically meaningless for close binaries in orbital
motion.
The summary of system types and the corresponding meaning of period and separation is given in the Table 1 (click here), where P is the true orbital period, a is the apparent semi-major axis, p is the parallax and M is the mass sum.
Table 1: Codes of system type and meaning of period and
separation
For each sub-system the visual magnitudes and spectral types of the components are given. These were collected from all available sources and are of very unequal quality. Evidently, MSC is not a photometric catalogue! However, the information on magnitudes and spectral types is essential in understanding the nature of the systems and in planning of observations.
Mutual inconsistency of component and system magnitudes is frequent in MSC because no special effort was made to adjust data from different sources. The distinction between individual and combined magnitudes is not always clear, although individual magnitudes were preferred for "single'' components and the combined magnitudes for unresolved multiple components. In some instances the individual component magnitudes were calculated from the combined magnitudes and the published magnitude difference. For some systems the yet unpublished area-scanner photometry is given (Shatskii 1996). If the spectral type in not known, e.g. for faint visual components, the B-V color is given instead when available.