How to use different line weights
in your drawings:
Here, the typical rule is the closer the object, the darker the line; the further away, the lighter the line. This is because read thicker, heavier lines as being closest to us, whereas lighter lines recede into the background. This is particularly important for making sense of perspective drawings, and for giving depth to 2D orthographic drawings - in particular Elevations and Section where space is flattened.
Importance & Hierarchy
Heavier, darker lines suggest more importance, so you need to be selective about what information you are showing in your drawings, and the line weights you attribute to different elements. This is a personal decision making process which can vary from drawing to drawing.
For example, you might draw construction lines and guides to help you set up the drawing and get other lines in the right place. While they are important for you while drawing, these lines are often not important for others to see, so were traditionally drawn very very lightly, and often erased later.
But there are no hard and fast rules here - you need to be selective about what is appropriate and important to help you communicate your project. Often at concept sketch design level, or in University project, you might choose to keep or even enhance these lines to communicate the process and add texture and atmosphere to your drawings.
Depending on the scale of the drawing, you may show more or less information about the specific elements and materials which make up a wall. Sometimes you may give no indication of materials at all, and adopt an outline-only style, or infill walls and other elements with poche.
Sometimes, it is useful or even necessary to indicate materials using a variety of lines in your drawings. Generally, a light material is given a lighter line weight, and heavier materials are given heavier line weights. It makes more visual sense to draw stone with a heavier lightweight than you would the fabric curtains.
Architects use a number of drawings which rely on 'cuts' to reveal things we would not normally see. The standard rule is that anything that is cut through to produce the drawing will be a very heavy line weight.
For example, a Section Drawing is produced by taking a cut through the building. Any element that is sliced through is this process will be darkest or heaviest. What is visible in the room beyond or on a distant will will be much lighter. A Plan is also a cut, so the elements which are cut, usually walls, will typically be darkest.
This rule is also moderated by the idea of Materiality above. Where you choose to show each of the different elements that make up a wall (the wall 'build-up'), you would consider the appropriate line weight for each of these individually. It is the density of these lines, located closely together on the page, which gives the weight to the wall as a whole.
Outlines, edges and surfaces
This rule comes into play when drawing 3-dimensional forms in axonometric, isometric or perspective.
The 'outline' or 'silhouette' of an object - where the object ends and the space around it begins - are typically treated with the darkest lines. Mid-weight lines are used for other edges, which denote a change in plane, but aren't set against the background. Light-weight lines are used for any detail, texture, or elements which are embedded within or on the surfaces of the planes.
Remember that line weights are most useful when a variety are used in relation to each other. This is related to scale. As you zoom in to parts of a project, you should manage the relative line weights within the drawing.
For example, in a 1:200 plan, a pane of glass might be demarcated by a single, light-weight line. At 1:50 scale, the glass might be two mid-weight lines, and at 1:5, 12 light-weight lines, with a mid-weight line on either outside edge.