Both of your drawings are beautiful! You said that you randomly varied the line thickness in the horse. I think I might see some of this randomness in the lost lines appearing in your horse's outline, specifically the belly, cheek, and lower hind legs. These parts of the horse feel confusingly insubstantial to me. In Stan's Drawing Basics Line Weight lesson, he suggests three strategies for light weight: distance, light & shadow, and importance. Perhaps you could use these strategies to help guide your use of light or lost lines? See https://www.proko.com/course-lesson/how-to-draw-with-line-weight . I see from your other postings that you've already done this exercise, but perhaps it's worth reviewing? I'm reminded of how Stan used a lost-line with intention to suggest "light bloom" when rendering the rhino's horn, which is bright against a bright background. See Drawing Basics / Demo - Light and Shadow Line Weight, https://www.proko.com/course-lesson/demo-light-and-shadow-line-weight @ time index 4:25 Stan does stress that there are no hard and fast rules in his demonstration. A lot depends on your chosen style, and making line decisions can be quite challenging and draining (https://www.proko.com/course-lesson/demo-light-and-shadow-line-weight), at 16:30, he says "Did you guys get braindead after a while? There's a lot of lines to think about. So much analysis... My brain just starts to hurt." Also, perhaps you might find some inspiration from Guptill's compositional use of line weight in "Rendering in Pen and Ink", as illustrated in this video: https://www.youtube.com/watch?v=SsRaRZvS2oI&t=480s I hope some of this helps. Good luck. You're a great artist!

I tried to convey the tension and twisting in body with my lines but it looks like I failed . Can you help me with that ?

First off I love these images. My father was a lifelong lover of horses and he made hundreds of paintings of them and this image reminds me a lot of his work. Second, I think the portrait already has a lot of line variation in it, and I think you are selling yourself short. I like the kind of work that has outlines emphasized, but that is just a personal preference. I know what you mean about lost lines though, I struggle with that myself and the only advice I can give is to think about where the light is coming from when you draw. That will tell which lines you can loose. Hope that helps.

Hi Martin, your drawings are very good and the way you add line weight is working, even though it gives a more stylised look to the final product. As some other comments have already stated line weight is a matter of design, thus the whole concept is very personal and is going to make more and more sense through drawing and practicing. That being said, there are some "ground rules" being analysed and demonstrated by Feng Zhu in the video below that may help you in your journey. The part that interests you is around the 32nd minute: https://youtu.be/22XYoenU-0c

I think it is really good.

I think it's just an issue of design, which is notoriously difficult to teach. You can use lines like Stephen Silver, or like Alphonse Mucha, or like Beauguereau. Some artists will use dark lines on the shadow side, others will use it on the light side. Some will vary the thickness, others not. I don't think it's something you learn like perspective, but more like style. Which means go find artists that you love and study their work to see how they use line weight

The drawings are very good anatomically and proportionally. I might add one more thing (I already see a couple of good advice comments) If you want to avoid the stylised look you shouldnt make the line the principal source of contrast. Yuo can still use the contour line but try to build the sharper contrasts in the mass shadows of the face for example. In the horse drawing I see the contour line being darker than the patches of shadow in the body and that flattens the drawing.

Drawing is very much like speaking: pronounce each syllable with the same volume in the same rhythm, and the result is boring. There are many ways to do it correctly, there are no hard rules, just rules of thumb: use heavier lines for heavier forms, forms in the shadow, forms nearby, forms that are important, softer lines for softer forms. Look at the work of comic book artists who work in ink, and practice in ink, because it is a dead-honest medium for practicing line quality, where you cannot cheat.

Hey Martijn, your drawings are nice. I wanted to explain the line weight thing to you, and I thought it would work better with an example. I hope you dont mind. Line weights are another tool we can use to help give our drawings dimension and depth. Its not a very difficult concept, just try to thicken your lines on the underside and shadow side of a form, then thin the line and even break it up on the light side. I hope this answers your question. Keep up the good work :)