Twenty tiny piles, eight foundations splitting half up and half down, and a 97% win rate: Grandfather is about as easy as double-deck solitaire gets.

Grandfather Solitaire deals out twenty tableau piles, each capped at two cards, and splits its eight foundations in half: four build up, four build down. It's not the same game as Tvete's Grandfather, which some also call "Grandfather," so don't mix up the two. This version is quite simple, with a 97% chance of winning. Two decks fill the layout, and the deal decides most games before you make a move.
Other solitaire games including Father Solitaire, Grandmother's, Sir Tommy, and Twenty are comparable to this one.
Grandfather moves fast in short bursts; Solitaire is slower and more patient, if that's the mood you're after.
If you run into anything odd or have an idea that would make the game better, please contact me.
Have fun!
8 foundation piles: The first four build up from Ace to King; the last four build down from King to Ace, each in a single suit.
20 tableau piles: Each pile holds up to two cards, and its top card can be played to the foundations or an empty tableau pile. Empty piles refill automatically from the stock. At the start of the game, each pile is dealt one card face-up.
Stock: Click to turn over 1 card to the waste. One redeal (two passes through the stock total).
Waste: The top card can be played on the foundations or tableau.
Foundation piles come in two types.
Foundation A has four piles. Any Ace can start a pile here, and each pile must begin with a different suit. A card can be added if it's one rank higher than the pile's current top card and matches its suit, so only the 10 of spades can follow a 9 of spades. A pile can hold no more than 13 cards.
Foundation B has four piles. Any King can start a pile here, and each pile must begin with a different suit. A card can be added if it's one rank lower than the pile's current top card and matches its suit, so only the 9 of diamonds can follow a 10 of diamonds. A pile can hold no more than 13 cards.
Once a card is placed on a foundation, it stays there.
Twenty tableau piles, each dealt one card face-up.
Cards from the waste can be played onto any tableau card, regardless of suit or rank, so any card can follow any other card. Cards can't move from one tableau pile to another.
Cards not covered by another card can be played onto the foundations or an empty tableau pile.
Empty tableau spots refill automatically from the stock. Once the stock runs out, any card can fill an empty spot.
Only one card may be moved at a time; sequences are not permitted.
There's a single waste pile, and the rest of the deck makes up the stock.
Click the stock to deal one card to the waste. There's one redeal (two passes through the stock total).
The top card of the waste can be played to the tableau or foundation.