Commands, attached pronouns, and the vosotros form

Commands, attached pronouns, and the vosotros form
 

In my workbook I'm doing some exercises on the command form with reflexive verbs and attached pronouns.

One of the questions asked me to give the command form of "acostarse" in the vosotros form with the pronoun attached. I thought that the answer would be "¡Acostados!" because the conjugation that I see in my charts says "acostad" for the 2nd person plural command form.

But the answer key in the book has "¡Acostaos!" Huh? Help - what am I missing?

Please recognize that I have been virtually ignoring the use of "vosotros" completely because I am almost singularly interested in Latin American Spanish. So I am quite ignorant of any special cases with vosotros or its conjugations...

Thanks!!

Acostar

Acostad 2nd person plural command - Acosta - 2nd person singular command
Acostaos 2nd person plural command with pronoun (os) - acóstate 2n person singular command with pronoun (te)

The (d) is dropped when adding pronouns to that form:) I believe it´s because if we kept the D some verbs it would appear to be a noun

cuidaos - take care of yourself (plural
cuidados - noun cautions


Hope it helps!

Thanks, Bob!

@Lou Ann: I'm bad for the "vosotros" thing, but that "d" seems to be dropped both because of euphony reasons and to avoid confusion with the "participio". :)

There is just one exception for the verb "ir": idos.
:)

Thanks all. ("Euphony" - I need to keep remembering that. We English-speakers obviously don't care about euphony!) :D