Perspective Grids is free to use and has no advertising inside the tools. It earns money in two ways: the printable and digital grid packs sold under the Perspective Grids name, and affiliate links in some of the software and lesson pages.
Affiliate links
When a guide about a piece of paid software links to where you can buy it, that link may be an affiliate link: if you buy after clicking it, the vendor pays this site a commission. The price you pay is the same as without the link. Affiliate links are marked with a short note beside them and carry rel="sponsored"; plain links to free software, documentation and downloads are not affiliate links.
The guides are written first and the links added afterwards. Where a free program does the job, the guide says so and links to it; where this site’s own tools do the job without any software, the guide says that too, which is often the case. No guide recommends a paid product because of its commission, and the software guides for Krita, GIMP, paint.net, Affinity and the site’s own tools contain no purchase links at all.
Programmes
Affiliate programmes currently in use: Huion Affiliate Program. The gear links on lesson pages go to Amazon.com through the Amazon Associates programme: as an Amazon Associate this site earns from qualifying purchases.
Products sold here
The grid packs on the templates page are made and sold by Perspective Grids through Gumroad; the tools that generate them stay free. Buying a pack is optional and changes nothing about the free tools.
Questions
Ask at hello@perspectivegrids.com. The privacy page covers what the site collects, which is separate from this.