CSM 166, Cantigas de Santa Maria (Alfonso X). From Medieval Spain.

T:Como Poden
M:6/8
L:1/8
C:Anon. 13th Century Spanish
N:CSM 166, Cantigas de Santa Maria (Alfonso X). From Medieval Spain.
K:C
"Refrain"zFF|FED F2G|A2 A2 AA|cBc d2 c/B/|A2 A2 AG|
FED F2G|A2 A2 AG|FED EDC|D2 D2 "Stanza"cc|cBG A2B|
c2 c2 cd|cBG A2B|G2 G2 cc|cBG A2B|c2 c2 cd|cBG A2B|
G2 G2 AG|FED F2G|A2 A2 AA|cBc d2 c/B/|A2 A2 AG|
FED F2G|A2 A2 AG|FED EDC|D2 D2|]

This cantiga (like so many) seems to have a varied rhythmic structure - I have used spaces in the ABC to define beats.  Every other bar the effective signature changes from 6/8 to a more steady 3/4.  Same length, but completely different rhythm.