Originating in the late fourteenth to early fifteenth century Spanish courts, the glosa is a delightful way for poets to exchange or build upon one another’s ideas in a structured poetic form.
A glosa normally has four ten-line stanzas preceded by four lines quoted from another poet (this quatrain also acts as a kind of epigraph to the poem). Each stanza ends with a line taken sequentially from the borrowed quatrain. While there is no required metre, lines 6, 9 and 10 of each stanza are often end-rhymed.
The glosa picks up on the concept of glossing – that is, elaborating or commenting on a text. Poets often vary the form slightly – for instance, by making some or all stanzas shorter than the standard ten lines.
One of the delights of a glosa rests in the process of writing toward the borrowed lines. The poet, in expanding on these lines, is working with something intrinsic to the other author’s words, something both share. This goes beyond technique, it’s like a curious marriage – two sensibilities intermingling.
(From In Fine Form – The Canadian Book of Form Poetry)