Spanish currency of the 1830s was a nasty little affair. The impracticability of its subdivisions is only surpassed by the wild profusion of coins and denominations. To add to the confusion, English authors such as Borrow and Ford often substituted English terms (‘Ounce’) for the original Spanish names (‘Onza’), or alternated prices in Spanish money with those in the old English duodecimal system of Pounds, Shillings and Pence. Finally, the exchange rate between Spanish and foreign currency was always reckoned in an imaginary coin, the Peso. The madness is therefore complete. It is more difficult to interpret a “note of bills” of a Madrid money-changer than it is to follow Max Planck on Quantum Physics.

There was no paper money in the 1830s, only coin; coin which was, in itself, in reasonable condition since the whole coinage had been renewed under King Carlos III around 1770 (Ford, R., A Hand-Book for Travellers in Spain, reprint of the 1845 edition, ed. by Ian Robertson, Centaur Press: London 1966, p.8). The following coins existed for real and ‘in the flesh’:

The Onza (Ounce in English) which equalled 320 reales
The Media Onza which equalled 160 reales
The Doblon which equalled 80 reales
The Dos Duros which equalled 40 reales
The Duro (a.k.a. Peso fuerte, or “strong weight”, and Dollar in English) was 20 reales
The Medio Duro was 10 reales
The Peseta (the “little weight”) was 4 reales
The Dos Reales was - as the name implies - a coin of 2 reales
The Real Vellum was the standard currency unit and equalled a handy 34 maravedís
The Dos Cuartos was 8 maravedís
The Cuarto was 4 maravedís
The Octavo was 2 maravedís
The Maravedí was practically worthless (± 0,03 reales) and barely existed as a coin.

The Onza, Media Onza, Doblon, Dos Duros and Duro came in gold. The Medio Duro, Peseta, Dos Reales and Real in silver. The dos cuartos, cuarto, octavo and maravedi in copper; or in whatever piece of scrap metal - chopped from old canon and recycled bell towers - struck the fancy of the market. Richard Ford tells in his Handbook, p. 8 (and again in chapter 9 of his Gatherings from Spain) how in the late 1820s, by way of experiment, he changed a gold Duro on the Seville market-place, and received in return an astonishing collection of odd coins, which included all sorts of ancient Spanish denominations, and even some antique Arab and Roman ones, generally assumed to possess the value of a maravedí (1)

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(Peter Missler: The Monetary System of Borrow’s Spain (1830-1840), footnote 1).
This old make-do habit took a long while to die, even if it lost a little in antiquarian appeal. In the early 1980s, it was still a common practice in Spanish shops, supermarkets and road-side stalls to return the client a few chewing-gums or sweets with his change if no 1 peseta pieces were at hand in the cash-register. .

So far things were reasonably orderly; but in practice, a number of legal and illegal variations ensured that chaos would reign supreme. Ford mentions (Handbook, p. 12), that there was a special issue of gold Media Onza, Doblon, Dos Duros and Duro, with a narrow thread or “cord” stamped around the rim, and called “de premio” for possessing a small additional fractional value. On p. 10 he warns that many of the standard gold coins, particularly the giant Onza, were worth a fraction less for having been heavily clipped, and were therefore rarely accepted at their nominal value (he actually advised travellers to acquire a legal certificate for each Onza they carried, made out by a gauger, and specifying each coin’s exact gold weight and value!) And on p. 9 he points out that there were still in circulation some 18th century “quarter dollars” (5 reales) and “half quarter dollars” (2,5 reales)... Small surprise that the Spanish peasantry during the Peninsular War actually accepted as payment counterfeit ‘gold dollars’ which the British soldiers concocted by flattening their their brass uniform buttons (Ian Robertson, A Commanding Presence, Spellmount Ltd: Stroud 2008, p. 293 note 24)! How could the poor sods know that no such legal tender actually existed?

Next to these real existing coins, there were also two imaginary monetary units meant only for paper transactions (see Ford, Handbook, p. 12):

The Peso, wholly imaginary for banking and exchange purposes, equalling 15 reales
The Ducado, merely used to talk about prices, with a value of 11 reales.

Throughout the 1830s, tourists, travellers and international bookkeepers (those of Borrow’s Bible Society among them) equalled 100 reales with 1 Pound Sterling for all purposes of calculation and accounting. This was a fictitious rule of thumb, mainly inspired by the impossibility of keeping adequate everyday track of an exchange rate between the Spanish system, in which the non-existent ‘Peso’ equalled 15 reales of 34 maravedís each, and the British coinage, in which 1 Pound Sterling equalled 20 shillings, each of which valued 12 pence (2)

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(Peter Missler: The Monetary System of Borrow’s Spain (1830-1840), footnote 2).
To make matters ever more incomprehensible, British bookkeepers and administrators in their bills and calculations sometimes call the imaginary 'Peso' by the name of 'Dollar' (symbol $), despite the fact that 'Dollar' really ought to have been exclusively reserved for the 'Duro' of 20 reales, otherwise known as the 'Peso Fuerte'... The confusion is understandable, but the effect is awful..
For this ideal, fictitious situation, the following table may be drawn up to show the relative values between Spanish and British coins:

	Maravedis	Reals	Peso-Dollar	Pounds	Shilling	Pence
Maravedi	1	0,03	0,002	-	-	0,07
Real	34	1	0,067	0,01	0,2	2,40
Peso-Dollar	512	15	1	0,15	3	36
Pound	3400	100	6,67	1	20	240
Shilling	170	5	0,33	0,05	1	12
Pence	14,20	0,42	0,028	0,004	0,08	1

Of course, in real life, the exchange rates fluctuated considerably. Throughout the year 1837, for instance (a year for which I happen to possess data), 1 pound sterling stood at 97,6 reales in February, rose gradually to 101,4 reales over the spring, reached a top of 102,5 in late November, and dropped back to a neat 100 again in mid December. All of which reflects a hefty swing in exchange rate of 5 % within a year.