But, you may say, a long metre is an impossibility: if it's longer than a hundred centimetres then it's a metre and a bit.
Of course in most worlds you'd be quite right, but it depends what you're measuring, and with long metre what you're measuring is hymns.
Now, the length of a hymn is a jolly useful thing to know if the bishop, for instance, isn't wearing the heels for sprinting; but as it happens long metre isn't to do with the length of the hymn from beginning to end, but with the length and arrangement of the lines.
An important consideration with hymns, you see, is not only how long they take to sing, but the tunes to which you can sing them. For this reason hymn metre is considered rather differently from poetic metre.
Long metre (or LM to the hipper sort of organist) tells you that each verse has four lines, that each line has eight syllables with alternate weak and strong stresses starting with a weak one (like the word away), and that the second and fourth lines rhyme.
The first and third lines might rhyme as well, but that's optional.
Armed with this information, you can sing, if you so wish, the words of O Come O Come Emmanuel to the tune of On Jordan's Bank the Baptist's Cry.
Or, alternatively, Hernando's Hideaway.
Thing To Sing Today: something in long metre. Or something not in long metre. The word metre comes from the Greek metron, which means measure.