To determine if you need a Front Rail Seal measure the difference in height from the truck's side bed rails (including, if any, bed rail caps) to the truck's front bed rail. For the cap's side base rails (when using a Front Rail Seal) install one of ESI's high quality 20ft truck cap seals (see choices below). To seal the cap's front and side base rails (not using a Front Rail Seal) install one of the following 20ft or 23ft length truck cap seals: 1 1/2" wide Super Cap Seal 23ft (ASIN: B00C7VU9BA) or Super Cap Seal 20ft (ASIN: B00NCC0SY6) fills all bed rail gaps, use on truck caps that weigh 200 lbs or less. 2" wide Premium Cap Seal XL 23ft (ASIN: B00IPQR63A) or Premium Cap Seal XL 20ft (ASIN: B00NCCX6C2) fills small to medium bed rail gaps, use on truck caps that weigh 200 lbs or less. 2 1/8" wide Super Cap Seal XL 23ft (ASIN: B00BNGBXOC) or Super Cap Seal XL 20ft (ASIN: B00NCCNUJQ) for trucks with extra wide bed rails, fills all bed rail gaps and for caps over 200 lbs. 1 1/2" wide Ultra Cap Seal 23ft (ASIN: B00MPMBIZW) or Ultra Cap Seal 20ft (ASIN: B00NCCDXJ8) fills all bed rail gaps, for caps over 200 lbs. IMPORTANT: To completely seal out dust and water that enter in through the tailgate gap and sidewalls, install a patented Ultimate Tailgate Seal with Taper Seal (ASIN: B00IPQWXZQ). MADE IN USA