A letter of support is a short signed statement from an organisation other than yours saying it will do something specific if the grant is made: host the sessions, refer the clients, provide the space, contribute the match. Funders read them for one thing, which is whether the plan described in the narrative has anybody behind it besides the applicant. That is why five near-identical letters signed by five different letterheads do more harm than three specific ones, and why the request has to go out far earlier than most offices assume.
What the letter has to say, in four sentences
Who the partner is and why their view counts. What specifically they will do if the grant is made, in numbers where numbers exist: how many referrals, how many hours of space, what value of in-kind. Why they believe the applicant can deliver, ideally from experience of working together. And a plain statement of support with a signature, a title and a date. Anything longer is read as padding; anything vaguer is read as politeness.
Why you will write most of them
Partners agree readily and then ask you for a draft, which is reasonable rather than lazy: they know their own organisation and not your programme. Plan for drafting most of them and you will be right more often than not. Draft each one differently, in that partner's voice and about that partner's contribution, because a reviewer reading five letters in a row will notice identical sentences immediately and will discount all five.
The timing, which is what actually goes wrong
Work backwards. A partner takes on average a week and a half to sign once asked, longer in July and December, and you will chase most of them twice. That puts the request roughly a fortnight before the deadline at the earliest, and the free sheet on this site works the exact date from your own partner count and turnaround. Letters requested a week out arrive after submission or not at all, and an application short of its letters is usually an application not submitted.
Support letter, letter of support, letter of commitment
These names are used interchangeably by most funders and precisely by a few. A letter of support says the partner endorses the work; a letter of commitment says the partner will do a specific thing, sometimes with a value attached. If the notice of funding opportunity uses one term, use that term, and if it asks for commitment then a warm endorsement without a specific undertaking will not count.
Questions people ask about letter of support
How many letters of support should an application have?
As many as the notice asks for, and otherwise one from each organisation genuinely involved. Three specific letters beat eight generic ones.
Can a partner write their own?
Gladly, if they will. Most ask for a draft, and giving them one you would be happy to see signed is the fastest route to a letter that says something.
Does the letter need a date and a letterhead?
Yes to both, and the date should be recent. An undated letter or one on plain paper reads as recycled, which is exactly what a reviewer is checking for.