It’s as broad as it is long
is a popular simile meaning there is no difference between two alternatives.

I’ll come back to length and breadth later.

First, second and third are the odd ones out.  All the other ordinal numbers end in TH.  Sometimes the number just takes the ending, other times there would be too many unpronounceable consonants and the spelling is altered for this reason.  Sometimes a vowel is dropped or a Y is changed to IE.

Fourth, sixth, seventh, tenth, eleventh, thirteenth to nineteenth inclusive and umpteenth (!) all retain their cardinal’s spelling as do hundredth, thousandth and millionth. Fifth, eighth, ninth, twelfth, twentieth to ninetieth are modified.  I’ll let you work out what the intermediate values are!

But what about other words in the English language ending in TH?  In some cases it seems to be an alternative to the ending NESS for making a noun from a verb.  NESS makes a noun from an adjective.

Health, wealth, stealth, dearth, growth, hearth, warmth, strength, length, breadth, cloth, tilth, youth, truth and ruth can all be related to (usually) shorter words with a similar meaning: heal, weal (well-being), steal, dear (a rarity is more expensive), grow, heart, warm, strong, long, broad, clout, till (cultivate), young, true and rue.

Other words include worth, filth, earth, bath, path, lath, plinth, firth, mirth, breath, birth, death, wreath, berth, broth, troth, pith, mouth, heath, sooth, forsooth, neath, underneath, beneath, with, smooth, faith, North, South,  forth, froth, myth, synonyms loth and loath, uncouth and the less common couth.  Unintentional pun!

Doth and quoth are archaic forms left over from the time when verbs were conjugated thus:

I go, thou goest, he/she/it goeth

I have, thou hast, he/she/it hath

Kith is always paired with kin; outwith is used in Scotland to mean without.  In a mouth, teeth are found and their singular is tooth.

The American English word math is always maths here and so does not really belong here!  Either way it is short for mathematics.

Many of these words add a Y to give an adjective: worthy, filthy, earthy, breathy and pithy.

Mirth is related to merry and perhaps a ferry could cross a firth.

  Firth of Forth bridges (Photo credit Wikipedia) http://en.wikipedia.org/wiki/File:Theforthbridges_fromdalmeny.jpg

Other forms exist in place names and include garth and wath.

A famous Bible story is about Goliath of Gath.  It can be found in 1 Samuel Chapter 17.

When other endings apart from Y are used with these words, they could lose the TH.
The only example I can think of is worship (worthship).  Can you think of any others?