Why the ottoman's angle matters
A footstool is flat. This is the front half of a lounger.
The seat of an Adirondack chair tilts back, so your knees are higher than your hips and your feet reach forward and down. A flat footstool puts your heels up and leaves your calves unsupported. The matching ottoman is cut at the seat's angle and set at the seat's height at the front, so it continues the line of the seat: your legs lie along it, the recline is complete, and nothing is holding you up but the chair.
When to buy one
If the chair is for reading, napping or an evening by the fire, buy it; it is the difference between a chair and a lounger. If the chair is for a dock where people sit for twenty minutes, or a porch where people perch, skip it.
Where it goes
Tucked under the front of the seat when not in use, so it takes no room. Pulled out, it adds about 18 inches to the footprint.
Price
$149 $79 USD, in the same color as the chair, from the same run. Add it on the product page.