Spider Solitaire uses two decks and asks for eight complete descending sequences, king down to ace, each of which is removed from the board when finished. Cards stack in descending order regardless of suit — but only a run of the same suit can be picked up and moved as a group.
That single restriction is what makes Spider the hardest of the mainstream solitaires. Building a mixed descending run looks like progress and produces a stack you can never relocate, which is how most games quietly become unwinnable well before you notice.
Because only same-suit runs move as a group. An off-suit descending stack looks like progress but can't be relocated, so it blocks the column permanently — which is how most games lock up.
One suit is the gentle introduction, two is the standard game, and four suits is genuinely difficult — most four-suit deals are lost even by strong players.
One card is added to every column, face up, regardless of what's underneath. It can bury carefully built sequences, so it's a last resort rather than a routine move.