Hmm....I think there may be a bit of a vocabulary problem here. I don't know anything about fifteenth-century Spanish rigging terminology, but the method of fastening a sail to a spar was pretty consistent throughout the sailing ship period.
In English, a clew of a square sail is simply a lower corner of it; a clewline is a piece of running rigging that hauls the clew up to the yard when the sail is being furled.
The head of the sail is secured to the yard (or gaff, or whatever) with a series of short lines called (in English) robands. There generally are two reinforced holes in the head of each individual cloth (the cloths generally being about two feet wide, or a little less); the roband passes through the hole and around the yard (sometimes twice), and has its ends tied together, sometimes with a simple reef knot and sometimes with a knot that R.C. Anderson, in his book The Rigging of Ships in the Days of the Spritsail Topmast, labels "impossible to describe but...easy enough to fit." The sail is furled (i.e., bundled up to the yard in such a way that the wind doesn't catch it) using a series of somewhat longer lines called gaskets. The gaskets are more-or-less permanently secured to the yard (or, in later centuries, the jackstay); when the sail is hauled up for furling they're passed around the sail and the yard and tied to themselves. I don't know how many gaskets there would have been on a given yard in a Spanish ship of the fifteenth century, but in later years the number varied between four and a dozen per yard.
A brail is a piece of running rigging used to haul up the foot of a lateen sail for furling.
Milton Roth's book, I'm sorry to say, is a bit of a sore subject with me; I don't recommend it. I don't imagine anybody's much interested in why, but here's a review of it that I wrote for the Nautical Research Journal in 1988:
"With the untimely death of Dr. Milton Roth ship modeling lost one of its most enthusiastic promoters. A retired podiatrist, Dr. Roth came to the hobby late in life but made a noticeable impact on it. He was a well-known, jovial fixture at NRG conferences, and his articles appeared regularly in several commercial magazines. His mail-order supply house, 'The Dromedary,' earned a fine reputation for fast and friendly service. Dr. Roth also acquired his share of critics, who wondered (and sometimes asked none too subtly) how thoroughly he really understood his subjects. This book probably will give his admirers and detractors alike about what they expected.
"In style and content it is a highly individual work. Irrespective of the title, the text deals almost exclusively with sailing ships; powered vessels are mentioned only in passing. The weight given to various topics seems to reflect the author's own interests. We get two paragraphs on the 'lift' method of hull construction and sixteen on techniques for removing CA adhesive from various parts of the human anatomy. Much of the commentary on the problems of product distribution will be of limited interest to most readers. On the other hand, Dr. Roth deserves applause for his forthrightness in describing the defects of imported 'plank-on-bulkhead' kits.
"Those who knew Dr. Roth will find his personality stamped on every page. Other readers will, on numerous occasions, scratch their heads as they attempt to figure out what the author was trying to say. (A representative sentence: 'It will be for you to decide what you have learned as distinct from what you have not already known.') The best parts of the book are the good-humored introductions to the subject's most basic elements. There are some peculiar errors of fact (no reliable evidence suggests that Samuel Pepys ever built a ship model, and the reference to the death of an English king in 1712 is bewildering), but most of the historical information is general enough to be innocuous. The lists of sources for tools and kits will come in handy, and the bibliographical notes, though they have a few holes in them, are reasonably thorough and up to date.
"The weakest chapter is the one entitled 'Size and Scale.' Herein, I fear, the reader will get lost in a jumble of outright misinformation ('1/4 inch = 1 foot' and 'quarter scale' do not mean the same thing), and murky syntax ('the popular scale, 1/4 inch = 1 foot, which we have established as also being equal to four feet of the size in the original object, is expressed in units'). 'To find the decimal equivalent,' we are advised at one point, 'divide 12 by the scale you wish to model.' Six lines later we read: 'To find the decimal equivalent, divide the scale by 12.' At this point I'm not sure what the author intended the term 'decimal equivalent' to mean, but one of those statements has to be wrong. An understanding of scale is basic to any form of scale modeling. Burying it under three pages of verbal and arithmetical confusion does no service for anybody - least of all the novice for whom, presumably, the book was intended.
"The text runs into trouble whenever it gets involved with mathematics. The author apparently wasn't at ease with the concept of percentage; he tended to write '.22 percent' when he meant '22 percent.' The chapter on 'Proportions for Rigging' ends with a table entitled 'Standing Rigging of the 74-gun Ship of the Line, Washington, 1815 (based on tables of USN 1826).' There follows a list of rigging lines, accompanied by a list of numbers. What do the numbers mean? Well, the explanation at the bottom reads: 'Ratio/Proportion of spar Diameter Circumference (In Inches).' Good luck, intrepid beginner.
"Many of the book's problems undoubtedly stem from the sad circumstances under which it was finished. If the author had lived to give the proofs a thorough reading he undoubtedly would have made some major changes. (I don't think he really meant to say, for instance, that 'there is no difference between a scalpel and a saw in their respective cutting abilities as long as they are sharp.') Part of the blame for the book's overall sloppiness must also rest on the shoulders of the publisher. Virtually every page contains grammatical and/or typographical errors. The manuscript needed, and apparently never got, the attentions of a competent editor.
"The book includes about 250 illustrations, the majority of them reprinted from other published sources. The drawings range in quality from the superb (George Campbell's perspective view of H.M.S. Victory's foretop) to the awful. In several cases they do not quite serve their intended purposes. A reprinted drawing captioned 'Buttock Lines in the Sheer Plan' also shows the station lines and waterlines - and provides no hint as to which are which. The captions generally acknowledge the sources properly, though the names of several artists and repositories are garbled. Two old prints are credited to 'The National Maritime Museum, Washington, D.C.' No such institution exists.
"There also are numerous photographs - but, surprisingly, none of Dr. Roth's own models. The quality of reproduction generally is high, the thirteen color plates being excellent. But at least one photo is printed in reverse, and another is upside down.
"The last chapter offers advice to the modeler who wants to turn professional. A rather emotional passage lays out a formula to determine the price of a kit-built model. Much of this discourse is predicated on the emphatic assertions that $3.65 x 50 hours = $175 and 100 x $80 = $800. Should the reader laugh or cry?
"Milt Roth was a first-rate gentleman, and I wish I could recommend his book. His many friends probably will value it as a poignant memorial to a likable, outgoing, and ebullient character. But it is not a sound introduction to ship modeling."
To be fair, Milt Roth had - and continues to have - his admirers. When I quoted that review in another web forum (Drydock Models), I was surprised at how many people jumped to the book's defense. (On the other hand, quite a few people in another forum, Modelshipworld, agreed with me.)
My personal opinion is that's it's one of the awfulest books about ship modeling on the market. But to each his/her own.