hendeca

 You can think of the Spanish 11 syllable line as an iambic pentameter, with a feminine ending, and a more "lilting" sound than in English:  

Los invisibles átomos del aire

The stress on syllable 10 is obligatory: 

Los invisibles átomos del aire

Syllables 7, 9, 11, cannot have accents (with a few exceptions).

Siempre la claridad viene del cielo.  [note stress clash here between syllables 6 and 7]. 

There is almost always a strong stress on 4 or 6.  If the stress is on 6, there will also be a stress on 1,2, or 3 (usually).  If there is a stress on 4, there will also be a stress on 8 (typically).  

We could see the structure as relatively free in the first four syllables of the line, and then resolving in a cadence in the seven syllables at the end:   

oXoooXo

This is also a typical rhythm for the 7 syllable line.   

So the line could be conceived of as iambic:

oX oX oX oX oXo

But it is more typically this:

oXoooXoooXo. [el dulce lamentar de dos pastores]. 

or this:

oooXoooXoXo

Flexible, flowing verse paragraphs can arise from the combination of lines of 7 and 11.  Note the tendency of accents to fall on the even numbered syllables. The enjambment, the unpredictable alternation of lines of 7 and 11, the unpredictable rhymes. This is the equivalently fluid verse paragraph that corresponds to English blank verse. (If you look at free verse in Spanish, you will see that it derives from this. All you have to do is add in some lines of 9 and 14 syllables and stop rhyming.  Then you have Vicente Aleixandre.)  

Piramidal, funesta, de la tierra                                4,6,10
nacida sombra, al cielo encaminaba                       2,4,6,10
de vanos obeliscos punta altiva,                            2,6,8,10
escalar pretendiendo las estrellas;                        3, 6, 10
si bien sus luces bellas,                                        2,4,6
exentas siempre, siempre rutilantes,                    2,4,6,10
la tenebrosa guerra....                                           4,6