record standards for the United States follow
the guidelines of the Recording Industry Association
of America (RIAA). The inch dimensions are
nominal, not precise diameters. The actual dimension
of a 12 inch record is 302 mm (11.89 in), for
a 10 inch it is 250 mm (9.84 in), and for a
7 inch it is 175 mm (6.89 in).
made in other countries are standardized by
different organizations, but are very similar
in size. The record diameters are typically
300 mm, 250 mm and 175 mm.
is an area about 6 mm (0.25?) wide at the outer
edge of the disk, called the lead-in where the
groove is widely spaced and silent. This section
allows the stylus to be dropped at the start
of the record groove, without damaging the recorded
section of the groove.
each track on the recorded section of an LP
record, there is usually a short gap of around
1 mm (0.04") where the groove is widely
spaced. This space is clearly visible, making
it easy to find a particular track.
the label centre, at the end of the groove,
there is another wide-pitched section known
as the lead-out. At the very end of this section,
the groove joins itself to form a complete circle,
called the lock groove; when the stylus reaches
this point, it circles repeatedly until lifted
from the record. On some recordings (for example
Spice by Eon), the sound continues on the lock
groove, which gives a strange repeating effect.
Automatic turntables rely on the position or
angular velocity of the arm, as it reaches these
more widely spaced grooves, to trigger a mechanism
that raises the arm and moves it out of the
way of the record.