Willie R. Meghar wrote:

Once upon a time (if memory serves) there was a discussion on saa
concerning the missing Pleiad.

The thread was called "Why Seven Pleiades?" -- you can look it up
in Google. Unfortunately, it splintered rapidly into a bunch of
side-issues
(partly my fault for a rather careless initial post), so it's hard to
sort the
substance out of the responses.

In my own experience, either 6 or 9 of the named stars can be seen
with the naked eye. If conditions are good enough to see 7, I'll also
see two more.

That's rather unusual. From the admittedly rather small sample of
responses, it seems that the most common numbers of Pleiads
seen in ideal conditions were six, seven, and eight. Nine wasn't
rare, but not much more common than ten.

Curiously, among those who see seven, it's pretty much a tie
between the seventh being Pleione (28 Tau) and Caelano (16 Tau).
Pleione is significantly brighter, but it's also a good deal closer
to its nearest partner.

Personally, I see six when wearing my normal glasses, with
optimal daytime correction, and eight when using an extra 0.5
or 0.75 diopters correction for optimal nighttime viewing. I've
never seen nine -- and not for want of trying.

Fast forward to today. I recently read about the Pleiades in yet
another book. An illustration showed the 9 brightest stars along with
their names. The text pointed out the seven sisters as well as the
two parents.

The assignment that we now use of specific names to specific Pleiades
dates from the early telescopic era, when people were already aware
that there are (at least!) several dozen Pleiads. It's unclear whether
the ancient Greeks assigned specific names to specific stars. But it
is pretty clear that they considered the stars that we now name as
parents (Atlas and Pleione) to be sisters, not parents.