American parties select their nominees through a sequence of state contests rather than a single national process. The rules differ by state and by party, which explains much of the apparent inconsistency.
Two layers of rules apply at once
Each state legislature sets how elections are administered within its borders, including whether a primary is held and when.
Each national party separately sets rules about how delegates are allocated and what a state must do for its delegates to be recognised at the convention.
Where the two conflict, negotiation follows, and parties have penalised states that scheduled contests outside the agreed window by reducing their delegate count.
Primaries and caucuses work differently
A primary is a state-administered election with polling places and ballots, conducted much like a general election but restricted to selecting a party's nominee.
A caucus is run by the party itself, requiring participants to attend at a set time and take part in a process that may involve discussion and multiple rounds.
Caucuses draw far fewer participants because attendance is demanding, and most states that once used them have moved toward primaries for that reason.
Eligibility to participate varies by state
Closed systems allow only registered members of a party to vote in its primary, while open systems allow any registered voter to choose which primary to participate in.
Semi-open arrangements sit between the two, typically permitting unaffiliated voters to take part while excluding those registered with another party.
These differences change who the electorate is from state to state, which is a substantive rather than a procedural distinction.
Delegates, not votes, decide the nomination
Contests award delegates to the national convention, and the allocation formula determines how a share of the vote converts into a share of delegates.
One party generally allocates proportionally above a threshold, while the other permits statewide and district-level winner-take-all rules in many states.
The same pattern of votes can therefore produce a very different delegate outcome depending on which party's rules apply, and on the geography of where the votes fell.
Sequence matters as much as arithmetic
Early contests are small but heavily covered, and results there affect fundraising, media attention and the willingness of donors and staff to continue.
Candidates who perform poorly early frequently withdraw before most states vote, so later electorates choose from a reduced field.
This is why the calendar itself is contested between states, and why the order in which contests are held is revisited by both parties between cycles.