Lead Grades:

The lead grade system revolves around two key aspects: hardness (indicated by the H) and darkness (represented by the B, for black). This scale spans from xH to xB, with x being any number. Although there's no strict limit on x, it typically hovers around 8, though there are extreme lead grades like 13B.

The middle ground, often denoted as HB, strikes a balance between darkness and hardness, making it ideal for daily use. Additionally, there's the F grade (firm), positioned between H and HB, which offers a good balance as well.

As you move up the B scale, you'll find pencils that produce darker lines, yet they tend to be softer and more prone to crumble, and are incapable of holding a sharp tip for a long time. Pencils above 4-5B take those negatives to the extreme, so I wouldn’t recommend them unless you have a specific use case for them and feel like no other choice is better. Conversely, descending the H scale leads to harder and firmer pencils, albeit they can be scratchy and may write lighter, posing potential visibility challenges. Again, don’t go too far in the extremes.

Exploring this spectrum of lead grades allows you to tailor your pencil choice based on your preferences, whether you prioritize the darkness of the lines, the hardness of the lead, or a combination of both. It's a nuanced world, and finding the perfect balance can enhance your writing or drawing experience significantly. Again, you might want to have multiple, so you have different ones depending on the situation you’re facing.

A rule of thumb would be to avoid lead grades above the number 6 from any side unless absolutely necessary, especially B, since they can write either TOO light and scratchy or can be too soft and crumbly as I mentioned earlier. 

Usually, most popular mechanical pencil grades only go up to a maximum of 4B or 4H (which is usually more than what you need), since the thinner lead can be harder to put together and isn’t very stable beyond 4B and 4H. 

It's worth noting that there are two primary lead grade systems: the European and the American. The above mentioned lead system was the European system, which offers a broader range of options, and is more widely used. Keep in mind that the American system does exist, with equivalents like how #2 in the American system corresponds to HB in the European system.

Lead Sizes:

If you’ve seen a mechanical pencil before, you’ve probably noticed a small number etched on them: like 0.5 or 0.7. These are lead sizes, and– you guessed it, determine the size of the lead. They are numbers measured in mm, and sometimes aren’t true to their name. 0.5 is actually closer to 0.6 mm, and 0.3 is closer to 0.4, and so on, except for a couple like 0.7, which is a bit more true to its name.

The most common lead sizes for MECHANICAL PENCILS are 0.3, 0.5, 0.7, and 0.9. If you’re wondering about the numbers in between, most of these lead sizes either don’t exist or are rarely used. The thicker ones (like 2.0) are generally found in clutch pencils (also called lead holders), and they tend to use thicker leads (2.0-5.6 mm) and usually hold one piece of lead at a time.

Here's a quick list containing an overview of each mainly supported lead size:

• 0.3 - The thinnest lead size that is usually supported by most pencils. Used mainly for precise applications like writing blueprints, graphing, or solving math equations. This lead, being very thin, is very fragile and can break with minimal pressure. Keep in mind there are some thinner lead sizes available, but aren’t supported by most pencils.

• 0.5 - The general lead size, perfect for anything from jotting down notes to writing long essays, and can do most of what a 0.3 or 0.7 does. It isn’t too fragile, but can still be broken with a bit of force. 0.5 is the most popular on the go! 

•  0.7 - Best for heavy-handed writers, can do what a 0.5 does but has a little more of a dull tip.

• 0.9 - A good thick tip, almost half of a wooden pencil's width, that is stronger and can be used in more different angles. Can still break under mediocre pressure though.

• 2.0 - Basically the size of a wooden pencil, usually used for what a wooden pencil does, but in a better and more customizable body, or for shading in art. 2.0 is a casual lead holder size.

Keep in mind, you NEED to have a pencil dedicated for each lead size. A 0.5 pencil won’t support 0.7, and so on. If you put different lead sizes than what is meant to be on a pencil, you can possibly jam and damage the pencil!

Exclusive Lead Sizes that are not commonly available:

• 0.2- The thinnest market-available lead size ever. Only Pentel makes 0.2 lead, and are only used in the Pentel Orenz Series. This could be good for compact equations, but it jams easily. Even applying minimal pressure can snap the lead.

• 0.4- The median size of 0.5 and 0.3. There are a couple of pencils that can use this, though they’re also pretty rare.