What the Interior Lines Mean

The outline, and then everything else

The outer boundary of a panel is the easy part: it is the object's overall extent in that panel's two directions, and nothing more. The top panel's outline is how wide and how deep the object is; the front panel's is how wide and how tall. Read it and you know the object's size in that view — and nothing at all about what is going on inside it.

Everything else the panel has to say is drawn inside that boundary. This is where Top-Front-End parts company with the keyholes section, which asks you to match an outline and nothing but an outline; here the inside of the outline is where most of the information lives.

TOP VIEW

FRONT VIEW

END VIEW

Everything is rectangles

These objects are built entirely from flat faces, each one parallel to one of the three panels. Nothing is sloped, nothing is rounded. That has a consequence worth leaning on: every edge in a view of the object runs either horizontal or vertical, so a panel is never anything more complicated than rectangles laid against and inside one another. There are no oblique lines to interpret and no curves to judge by eye — reading a view is reading rectangles.

What a line inside the outline means

A line drawn inside the outline is not a scratch on the surface and not the border between two parts. It is a change of depth along the direction you are looking — the edge where the object steps toward you or away from you. A step, a shoulder, the floor of a groove, the top of a post: from the front they all read the same way, as a line where the distance from your eye jumps.

So a rectangle sitting inside the outline is a face standing at its own depth. It is not a separate piece sitting on the object and it is not a hole through it; on its own it does not even tell you which way the step goes. What it does tell you, reliably, is that the surface changes at that boundary.

In the top panel of the object above, two small rectangles sit inside the plate's outline. Each is the top face of an upright post, standing at a different height from the plate around it — one solid rectangle for each, drawn where the surface steps up. Whether a given line comes out solid or dotted is the previous topic's business; either way, it is a boundary between two depths.

Reaching the edge, or not

Now the distinction that decides where a feature lives: does it touch the outline or not?

• A feature drawn wholly inside the outline, touching no border, sits in from every edge of the object in those two directions. The two post footprints above are like this — the outline of the plate runs all the way around, unbroken.
• A feature that interrupts the outline — leaving a gap in a border, or making it step in or out — reaches the outside of the object on that side.

TOP VIEW

?

FRONT VIEW

END VIEW

Look at the right-hand border of the top panel in this second object. It is drawn from the bottom corner up to a point, then stops, then picks up again higher, with a band running inward from the gap and stopping in mid-air part-way across. That gap is not a drafting slip. It says the cut is open at that end of the object and closed at the other. Had the same band been drawn with the border running whole behind it, it would be a feature buried inside, nowhere near the end.

A border that runs unbroken is a promise, and it is worth checking before anything else.

Count the rectangles first

Before you judge whether anything looks too wide or too tall, count. How many distinct rectangles does this panel divide into? The top panel of the first object divides into three: the plate's outline and two smaller ones inside it. That is a fact about the object you can state in a second, it survives every question about proportion, and it is much harder to get wrong than a size judgment.

Counting first also stops the commonest way to misread a panel — deciding what the object is from its outline, then reading the interior lines as confirmation. Take the rectangles as they come, then ask what arrangement of faces would produce exactly that many.

The takeaway

The outline gives you the object's extent in the panel's two directions and nothing else. Every line inside it marks a change of depth, and every one of them is horizontal or vertical, so a panel is always a set of rectangles. A feature drawn wholly inside touches no edge of the object; one that interrupts the outline reaches the outside on that side. Count the rectangles before you judge a single proportion.