Reading a spec sheet like a collector: five lines that tell you more than the marketing copy ever will.

Most spec sheets are honest about what they print and silent about what they leave out, and the silence is where the useful information hides. A collector reads a sheet in two passes. First the numbers, then the blanks. Both passes take about two minutes once you know what each line is for.

Start with the blanks

Look for the lines a brand has left empty before you look at the ones it has filled in. The Atlas entry for the Seiko Prospex Monster SRPD27 shows why. Its case is 42.4mm and 13.4mm thick, but lug to lug and lug width are marked “not published” in our verified sources. That one gap decides whether the watch suits a smaller wrist or a strap swap, and Seiko’s page does not settle it.

A blank is not a flaw in the watch. It is a task for you: measure it in a shop, ask the seller, or find a photo with a ruler in it. What you should not do is guess. If a number matters to your decision and you cannot find it, the honest answer is that you do not know yet.

Case diameter is the number everyone quotes

Case diameter measures the width of the watch face. It is the figure printed first and remembered longest, and it tells you the least about how the watch will sit. For that you want lug to lug, the distance from the tip of the upper lugs to the tip of the lower lugs, per Teddy Baldassarre. Lugs are the protruding parts of the case that hold the strap.

The same source compares two watches. A 45mm Seiko Turtle has 47.7mm lug to lug, and a 38mm Hamilton Khaki Field has 47mm. The diameters are seven millimeters apart. The wrist space they take is less than a millimeter apart. The big watch wears smaller than its diameter suggests, and the small one wears larger.

Our own Atlas shows the useful range. The Casio F-91W is 35.2mm wide and 38.2mm lug to lug. The Orient Kamasu is 41.8mm and 47.0mm. The Seiko SPB143 is 40.5mm and 47.6mm, so it takes more wrist than the Kamasu despite a smaller case. If your wrist is narrow, lug to lug is the line I would read twice. Lug width, the gap between the lugs, only decides which straps fit, which lesson 3.3 covers.

What the movement line and power reserve promise

The movement line says what runs the watch. “Quartz” means a battery. “Automatic” means a mechanical movement wound by your wrist motion, and “hand-wound” means you turn the crown. Lesson 1.1 covers the difference; here we only need to read the line.

Power reserve is how long a fully wound mechanical watch keeps running if you take it off. The Seiko NH35A, a movement that appears inside many smaller brands’ watches, lists 41 hours, 21,600 beats an hour, 24 jewels, hacking seconds and hand winding. The same page gives Seiko’s accuracy claim as minus 20 to plus 40 seconds a day. That is a real range and a wide one. If your watch gains half a minute daily, it is inside the claim.

Compare reserves across our entries. The Monster runs about 41 hours, the Kamasu about 40, the Seiko SPB143 70, and the Hamilton Khaki Field Mechanical 80. Forty-one hours means a watch left on a Friday night may be stopped by Sunday morning. Seventy or eighty gets you through a weekend and then some. Quartz watches run on a battery rather than a wound spring, which is why the Casio’s entry reads “not applicable” and quotes battery life instead.

A note on names. The NH35A is made by Seiko Instruments, which Caliber Corner lists as “aka” Time Module Inc., so a spec sheet that says “TMI NH35” and one that says “Seiko NH35” are likely describing the same family of movement. I could not verify a Miyota 9015 comparison this run, so I will not make one. When a sheet names a movement, look up its own spec line and compare reserve and accuracy claim.

What 50m water resistance means on a watch

This is the line most misread. A marking of 30m, 50m, 100m or 10 bar is a test pressure expressed as a depth. It is not a diving depth, per Wikipedia’s entry on the water resistant mark, which says a depth indication “does not mean” the watch was designed for repeated long term use at that depth. The current standard for these ratings is ISO 22810:2010, which replaced the older ISO 2281. The same source says that older standard was conducted only once, using static pressure, on a sample.

So 50m means the case passed a pressure test equal to that depth. It does not mean you can swim at 50 meters, and the same source says a 30m watch cannot be expected to cope with a long spell in a swimming pool.

Diver’s watches follow a different standard, ISO 6425. It tests at 125 percent of the rating, so a 200m watch is tested to the equivalent of 250 meters. It also adds thermal shock, a 50 hour immersion test, magnetic resistance, shock resistance and saltwater corrosion testing. That is why “200m, diver’s” in the Monster’s entry means something that a bare “200m” does not. My rule: for splashes and rain, 30m is fine; for swimming, I want a rating well above 50m and the word diver’s before I trust it.

Sapphire, mineral and acrylic on one line

The crystal is the clear cover over the dial. Three materials turn up. Sapphire rates 9 on the Mohs scale, is very scratch resistant, but is not scratch proof and can crack on a significant impact and cannot be restored once damaged, per Monochrome. Mineral glass is about 5, with Seiko’s Hardlex described as reaching 7, scratches in daily wear, and costs little to replace. Acrylic scratches easily, but it polishes out and does not shatter.

Price is the quickest clue. Mineral glass sits at the Monster’s $396 to $459 band, where I find it hard to defend. Sapphire is on the $238 to $308 Cronos L6033, a cheaper watch. Hardlex is a trade-off, not a scandal, but it is worth a pause at that price.

A worked example: three spec sheets side by side

Take three Atlas entries and read only five lines each:

• Casio F-91W: quartz, 30m, resin/acrylic crystal, 38.2mm lug to lug, $18 to $25. A cheap, honest tool.
• Hamilton Khaki Field Mechanical: hand-wound H-50, 80 hours, 50m, sapphire, 38mm. Strong reserve, modest water rating.
• Seiko SPB143: 6R35, 70 hours, 200m diver’s, sapphire, 47.6mm lug to lug, $1,100 to $1,200.

The Hamilton’s 50m is the surprise for a $500 to $595 watch, and the sheet tells you so before a salesperson can talk around it. Hamilton does not publish a lug to lug figure, so the 47mm above comes from Teddy Baldassarre, a third party. Even so, the SPB143 has the longest lug to lug of the three, so I would try it on first. Reading the sheet does not make the decision for you. It tells you which questions to ask in the shop.

Questions worth asking before you buy

What does 50m water resistance mean on a watch? It means the case passed a pressure test equivalent to 50 meters. It does not mean you can swim or dive to that depth, per Wikipedia.

What is lug to lug on a watch? The distance from the tip of the upper lugs to the tip of the lower lugs. It shows how much wrist the watch occupies, which diameter alone does not, per Teddy Baldassarre.

What is a power reserve on a watch? How long a fully wound mechanical watch runs without being worn. The NH35A lists 41 hours.

Is sapphire always better than mineral crystal? For scratches, yes: sapphire is 9 on the Mohs scale against about 5 for mineral glass. Sapphire can still crack on a hard impact, per Monochrome.

What should I do if a spec is missing? Treat it as unknown. Measure the watch in person or ask the seller before you buy a strap or trust a size.